I’ve had a lot of trouble with constant dropouts of cell module communication, I believe it’s my noisy inverter messing things up badly.
I notice communication is fine if inverter not running.
So I went back to basics and simply grabbed both the rx and tx lines, and controller starts picking up the modules right away!
Thinking I must be grounding the noise signals, I’ve next tried putting some aluminium tape over the rx and tx lines and connecting that to battery negative. Seems to be working well 
(Basically I’ve just made diy screened cable! 
)
Hope this helps people with cell coms issues?