Is there a way to switch between T-Mobile and AT&T on Boron 404x using a command line, adjusting the code, etc? I noticed some connection issues with T-Mobile, AT&T seems to have a better signal here. I'm in the US, thank you 
Unfortunately it's not possible to steer the connection to a specific carrier. It's determined by the cellular modem and the SIM card and you can't override it in software.
It may be possible to deny a specific carrier by MCC/MNC in the cellular modem using AT commands, but this is not recommended, it may vary by modem, and there is no example code available for doing it.
I thought that getting Boron LTE-M (NorAm) with EtherSIM (BRN404X) would help me choose between T-Mobile and AT&T with SYSTEM_MODE(SEMI_AUTOMATIC) but maybe it will switch on its own when T-Mobile fails, but so far it hasn't. Thank you.
If one carrier is down or unavailable it will switch to the other. The exact algorithm is a mystery, and it generally works, but in some cases where there is some signal on both, it will sometimes pick the wrong one, but there isn't a good way to override what the modem is doing.
Would it be possible in the near future to modify the firmware so that we pick between AT&T and T-Mobile? I noticed a significant difference between both carriers, my old Boron 402 runs on AT&T and the connection is steady, the new Borons 404X when it runs on T-Mobile I get multiple disconnects 
, it's just not good in my area. 
No, it's not possible to set a manual preference list for available carriers.