I brought my AT&T branded Note 5 with me to T-Mobile. I know that I don't get software updates without an AT&T sim, but would i still get security updates/patches?
You'll still rely on AT&T for the updates. You'll still get them too.. your phone will phone home in a sense, so it'll pull the right image for your phone. You don't need the AT&T SIM in it to get them.
@LeoRex, I always thought the phone had to be on the actual carrier network to get the appropriate system update. Have you been able to do it without being on the original network?
I've had a few phones.. some cheap prepaid Motos I got at Black Friday deals and the two Zmax's my kids have now... Maybe you get pushed to the back of the line. Dunno