This would sound like WiFi interference, but you have given no details to go on.
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.
However, other factors should be investigated first. When this happens, what are the lights showing on the front of the router? Do you have any wired connections & if so, how do they perform?
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?
