Speed on LB1120 Modem

Have LB1120 with Verizon Sim Card.  Getting constant 4-5 green bar constant with external antenna.  Also have GL-iNet Travel Router, Model GL-ar750s-EXT.  The router shows both connections which I can connect too.

The issue is the speed.  I have 0.5-0.8 Mbs download and about 0.6 upload when any router is connected to this modem.

I have two other routers which I have gotten great speeds (30-50 MPS) and replaced on this same LB1120 modem when all three went down to the same slow speed of about 0.5 MPS.  So that tells me it is not the router - I would assume.

I'm not a Tech guy so please explain in detail if you know of a fix or where I might be able to go to get to decent speeds.

Thanks for your support,