As you can probably guess from the replies so far..this isn't going to be straightforward. Home routers, especially the ones supplied free by ISPs, aren't all that great on wifi performance in general (one of my friends who's just moved into a house and got all new stuff from Virgin was advised
by them to buy her own wifi router and plug it into theirs!).
There are so many variables in wifi it's almost impossible to give a diagnosis without coming to your house - Wifi Analyser as mentioned above is a great tool for seeing how the signal strength dies off in various locations. 5G band wifi (n/ac class) is faster but dies off with range faster than the (slower) 2.4 GHZ a/b/g class ones - a lot of routers have both but it can depend how ready your device is to switch between them as you move around, etc. If you're getting -80db or less on the Wifi analyser chart it probably won't work at all.
We also have two routers in the house - for a couple of reasons, one was to give wifi to the bottom ot the (100ft) garden and the other was so that the kids would have their stuff on one and we would have ours on the other, so theirs could be de-activated at night to stop them messing about with their gadgets instead of sleeping (yes I am that Dad, every child's nightmare, that knows how all this stuff works!).
Router location has a huge effect on range, a lot of people put theirs on the TV stand or window sill downstairs. Our downstairs one is on a shelf up 7ft high in the understairs cupboard, the kids one upstairs is on the back bedroom window sill facing into the garden. Higher is better - the titchy 3ft lead you get given to go from the phone socket/filter to the router is the biggest blocker here, mine is on a 10m shielded cable (RJ11 plugs both ends if you want to look for one) thus the router can go anywhere within a 10m cable run. The upstairs one is connected to that with a 10m eithernet (RJ45) cable run behind the skirting, as is the Xbox upstairs (always better to wire stuff that isn't going to move, gets it off the wifi, and it's faster for gaming). If I was having a house built or re-wired I would have the whole lot flood wired with CAT6 Ethernet and a couple of sockets in every room....not many people have that option though!
The upstairs one is configured as a wireless access point (not hard to do, on some it's just a tick box in settings), and for the kids also had keyword filtering and time rules (off at night) applied. The signal reaches the bottom of the garden....except for a brief dead point just outside the back of the extension, due to the amount of walls in the way etc.
Modern houses can be much worse for signal blocking, the cavity wall insulation may have foil either side, and modern heat reflective double glazing has a very thin film of metal sprayed on to it to reflect heat, if you have both of those the house could practically be a Faraday cage and block all kids of signals. Water pipes and the material of construction also have a bearing (an older house with thick stone walls is equally bad).
Going back to the ISP supplied routers......the theory states that you should be able to connect just over 250 devices to one router (using IPv4.....IPv6 is witchcraft and I'm not going there
) however they seem to crap out at about 15-18, they just don't have the processing power to cope (the DHCP process for allocating IP addresses to each device seems to fox them) . Purchasing your own decent one may end up being the only cure (so just using the ISP one as a modem).
The Wifi calling issue is an odd one, though could be the phone itself? I've not long upgraded from a Sony Xperia Z3 to a 5iii, and the new one holds Wifi calling fine, the old one just would not, nothing else has changed (I think the old phone struggled with its radio though, the bluetooth would crap out and need a reset before it would work again).
Re the Asus laptop - it might be set to only connect to 5G, not 2.4? Check properties of network connection, though would be unusual for it to come from new like that.
Ask away if anything else, my knowledge is mostly experience but I do work in IT and have done both CCNA and CompTIA courses along the way