You get what you pay for. TT is priced very competitively, but when things go wrong, they are not the best at sorting the problems. I use a small company that charge £16 a month. On the two occasions where I've had a connection problem, it's been sorted in under 5 mins by an engineer in Cheshire on a local rate call.
UK's best mobile network 2016: best 3G, best 4G, best mobile networks revealed