I have a Mikrotik LtAP mini with a TMobile SIM and an AT&T SIM, and I need to figure out a script that can determine which connection is better and toggle to that SIM. I know the SIMs on this model can't be active at the same time since it is one modem with 2 slots.

The scenario is we send this out with an installer and would like it to lock onto the best connection and make that sim active. This only needs to be done at reboot and not an active thing that is always going.

I know I can do something like:

/int lte info [find where name="lte2"] once do={:set $netwType $"signal-strengh"}

Any scripting ideas to toggle to the SIM with the better connection?

LtAP mini have one sim slot then lte1 interface, not lte2 but in script you can use always [find].
To properly select a sim you can think about few parameters like
ping
speedtest
bands/width/2CA
ISP limit in GB or Mbps
via receive SMS from ISP
.... etc.

Means you can think about many options to determine the proper ISP and then, just select proper slot who WIN this selection's.
As base you can use https://wiki.mikrotik.com/wiki/Dual_SIM ... l_settings but of course the scripting you can write in your own way.