Then all you can do is go with the information you already found out, that the speed to the ONT is fine & the problem area is from there to your devices, via your router.
You said that it did not make any difference which WiFi channel you tried, but have you examined the levels of Wi-Fi interference on your own network? If you are connecting via the 2.4GHz band then there is a fair chance that the channels on this band are too congested to get the best speeds that your WiFi hardware can deliver. In that case, you might have to consider using only the 5GHz band, but not all your devices may support this newer band.
The most important thing at the moment is not to throw money at a situation unless you have proved what the problem is. This is where I can normally help TalkTalk customers for free.
Slow speed, intermittent dropouts, breaks in the signal, no signal on some or all devices or no internet access might be caused by Wi-Fi interference from other local networks, which can also lead to a permanent reduction in speed. No ISP can be responsible for your local environment, this is mainly a byproduct of the popularity of Wi-Fi.
Generally speaking, the 2.4GHz band suffers much more from interference than the 5GHz band, but the 2.4GHz one can sometimes have a better range, but this all depends on your local area.
If you would like me to help you, please can you specify the make/model of TalkTalk router that you are using?
