Rain 5g dismal speeds

I've had a a rain 5G Sim router now for a few months and when I received it all was perfect the speeds were good and didn't have a day of problems, I have not moved the position of the router once. But over the last 2 months the speeds have been shocking, I've contacted them and all they can tell me to do is move the router around until the signal is better, I've done that with no Improvement. How does a service decline so quickly.