Ok, as I understand it then your Panasonic telly has a built in freeview receiver and now it can no longer access iplayer. That's not actually uncommon as the BBC has updated its interfacing software a number of times over the years. It's up to the telly manufacturer to update their software to keep the facility and obviously Panasonic have decided its not worth the effort on your 'old' telly.
People also tend not to realise that when they buy a 'smart' telly (i.e. one connected to the internet) that they have to assure themselves that their telly will receive all the channels they need via the telly's internet connection. It is not uncommon (or certainly was not uncommon) for certain services to be licensed to one manufacturer only. I will give one example, when we bought our Samsung telly we did so knowing that it was the only telly around at the time that had channel 4's catchup service on it's 'smart' interface. No other telly gave that - 'yes' you could get a lot of other channels but not all of them as Channel 4 had not licenced its software to anyone else at that stage.
You say that iplayer has also disappeared from your freesat service. We use a separate Humax freesat box and hence my comments above on our installation.
Chromecast is a technology where whereby you connect a Chromecast wifi receiver to your telly via one of its HDMI inputs, it's looks a bit like a usb 'stick' and indeed with our Chromecast receiver there is also a flying lead on the 'stick' which you also have to connect into a usb port on the back of the telly as this is where it gets its power from. You then connect the 'stick' to your wifi router using the software supplied.
If I want to watch BTSports I cannot get it on my telly (freesat or freeview), neither is it available via the smart (internet) function on the telly as there is no suitable Samsung app. So, I open up BT sports app on my iPad, and open up and select the match I want to see. When I see the match (live on my iPad) I click on a 'cast' symbol on the top of the screen and the pictures and sound are then 'cast' to the telly - which you obviously switch to the pertinent hdmi input. Hey presto I am now watching the match on the telly. In reality I am not 'casting' anything from the iPad to the telly (its all done between the Chromecast unit plugged into the back of your telly and your wifi router) but in practice that's what it looks like to the user.
So, as well as having the Chromecast hardware you have to ensure any channel or material you want to see on your telly via the Chromecast app on your iPad/laptop can actually be 'cast' to your telly in the first place, for example, I believe youtube can be cast from a laptop to the Chromecast device. But can the BBC iplayer? Presently I am not at home and don't know the answer to that one. Some research on the www may tell you.
I believe the (similar) Amazon Firestick is a very similar device to Google's Chromecast and may also allow streaming (casting) of the BBC iplayer.
So, I suggest you have a search on the www and investigate to see what device will fit your telly and give you the additional channels you would like if that's a route you'd like to consider.
And yes, 5 years is old in telly terms I'm afraid.