DSL is pretty complex, here are some things I would do. It is not a troubleshooting guide as such but may give you some ideas.
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Test without WiFi, use an ethernet cable.
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Check that you have correct wiring of filter(s).
Why you need ADSL filters (microfilters)| Help | BT.com Help
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With everything unplugged from the phone line do an online BT line test. Anyone can do this for you. Just put the phone number into the web page. You of course cannot use your own DSL since it is unplugged:-) MAKE SURE YOU CAN PUT IT ALL BACK CORRECTLY.
https://www.bt.com/consumerFaultTrac...g.do?pageId=21
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Open the Master socket (has a horizontally split facia and the bottom half has two screws). There should (must I think) only be one on the line. Plug your router into the normal BT phone socket that you will find inside. I think it's usually on the right hand side. Do not use any extension cables or other rubbish on the line to the router:-)
This removes all premises wiring and devices from the system. Try it out. Try a different cable and filter. Ideally if you have an old modem--phone_socket cable with an RJ-11 on one and and a BT plug on the other use that, this allows you to leave out the filter. If it is an internal device causing the problem unplug progressively until you find the culprit.
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The diagnostics here may be helpful. I don't have a BT line so I can't go very far. It may just end up with a line test and a speed test but there was mention of extraction of BRAS setings.
BTW Performance Test
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Check the sync speeds etc., on the router. Post all of the line details here.
Screenshot is good.
The information looks something like this. This is from a Cisco which I happen to have a note of handy but all routers that I have seen have a similar page(s).
DSL Mode: ITU G.992.1 (G.DMT) Annex A
ITU STD NUM: 0x03 0x3
Vendor ID: 'STMI' 'TSTC'
Vendor Specific: 0x0000 0x0500
Vendor Country: 0x0F 0xB5
Chip ID: C196 (0)
DFE BOM: DFE3.0 Annex A (1)
Capacity Used: 83% 51%
Noise Margin: 11.0 dB 27.0 dB
Output Power: 18.5 dBm 12.5 dBm
Attenuation: 23.5 dB 10.5 dB
Interleave Fast Interleave Fast
Speed (kbps): 8192 0 448 0
Cells: 66780345 0 188102394 0
Reed-Solomon EC: 1797 0 0 0
CRC Errors: 28 0 0 0
Header Errors: 19 0 0 0
Total BER: 2992E-12 0E-0
Leakage Average BER: 1704E-12 0E-0
We want as much as possible but up and down, speed, noise margin, attenuation, and error rate are critical, if available.
Above by the way was a pretty good line with a very low error rate.
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Make sure no one else or no other process on your computer is using the line.
You could compare the Windows Task Manager/Networking rates with the speedtest rates.
You are using WPA2 on your wifi?
Your family is not downloading movies?
Most routers keep a list of all the computers that have been ussued DHCP addresses. Might be worth a check to see if there is unauthorised use.
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Modern DSL (I am a wee bit out of date) monitors the line for errors over the long term and slowly adjusts the connection speed over time. It is all done at the provider end. You need to get Sky to tell you what is going on with your settings. Switching off the router at night for example can fool the system into thinking that you have a bad line and to it cranking down the speed. To fix this you need to get the provider to reset it. BRAS?
Read the plusnet guide linked below.
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Try a different router. Sadly many of the free routers now are locked to one provider. You will have to put your login and password into the the router. The login usually looks like an email address and indeed confusingly may be or may not be an actual email address,
[email protected] was one form I saw recently. Something like 00192D3DB123@skydsl came in one I got from a charity shop. I think I had to do a bit of fiddling (software only) to un-Sky it.
Where do all the old routers go? The charity shops used to have loads of them but they have none at all now.
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Run the speed test and during the download test run in a command window
tracert -d 8.8.8.8
post the results. Repead for upload test.
What you want to see is much of the delay being generated in the second hop. If significant delay is being added later then there may be a problem in the providers network. This is very difficult to read if not used to network troubleshooting so don't get too excited about apparent anomalies. I have added some interpretation at the ends of the lines. < means less than.
C: \Users\fred>tracert -d 8.8.8.8
Tracing route to 8.8.8.8 over a maximum of 30 hop
1 5 ms 6 ms 2 ms 10.xxx <2ms - WiFi hop
2 20 ms 11 ms 17 ms 192.xxx <11 - DSL hop
3 15 ms 14 ms 16 ms 192.168.240.2 <14
4 14 ms 12 ms 13 ms 192.168.134.2 <12
5 13 ms 14 ms 12 ms 192.168.71.209 <12
6 14 ms 16 ms 13 ms 192.168.69.252 <13
7 18 ms 28 ms 44 ms 192.168.195.252 <18
8 19 ms 40 ms 17 ms 217.39.0.2 <17
9 * 23 ms * 192.168.2.1 <23
10 34 ms 27 ms 28 ms 217.41.219.192 <27
11 85 ms 163 ms 31 ms 109.159.249.103 <31
12 48 ms 21 ms 32 ms 109.159.255.197 <21
13 30 ms 28 ms 101 ms 194.72.31.81 <28
14 52 ms 28 ms 30 ms 109.159.253.67 <28
15 32 ms 18 ms 29 ms 216.239.47.179 <18
16 * 22 ms 23 ms 216.239.47.29 <22
17 27 ms 31 ms 26 ms 8.8.8.8 <27
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Some links:
Broadband Terminology Guide
This is pretty good.
Internode :: Support :: FAQ :: ADSL :: ADSL Troubleshooting
samknowsbroadband has good stuff.
BTW Performance Test
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Deary me I must be bored this afternoon.