Wireless Joint

Quectel AT commands are different than the Sierra and what I gave you might not work with it. You can get the entire AT command set from: https://www.quectel.com/UploadImage/Downlad/Quectel_EM12&EG12&EG18_AT_Commands_Manual_V1.0.pdf

Maybe someone else can give you better advice on which would be ...

Two ways of seeing your connection. You can go to Modem>Network Status>Signal information or you can go to Modem>Miscellaneous and give the AT command: AT!GSTATUS?

Also in Miscellaneous there is Cellular Mode where you can select 4G only.

Your cell tower may be different than 'others' can you get better speeds with a cell phone in the same location using ATT? Your tower may be congested, or the em12 may be sticking to a band that is saturated, even though it may be the strongest signal.