Nighthawk M1 MR1100 loses signal when external antenna connected

I had similar situation with MR1100-2A1NAS (with or without external antenna) which started summer of 2002. Rebooting the hotspot would resolve the issue but only for a couple of hours. Some folks speculate that transparent AT&T DNS proxy servers cause the problem which can be fixed by using secure VPN connection (like NordVPN) to bypass (see https://forums.att.com/conversations/other-phones-devices/att-transparent-proxy-not-stable-hanging-connections-to-websites/5ee7bfdf483393542c96b341). I tried this and it seemed to work better but has side effect of slower speed via encryption, multiple hops and subscription cost. It also requires having a router in line that supports OpenVPN (UDP) and creating outbound routing rules for streaming/gaming apps that don't like VPN (like Amazon Prime).

However, I read a post on AT&T community that a new SIM card fixed the problem for one user. I ordered a new SIM card from AT&T and PRESTO, everything started working again with a 20 Mbps increase in download speeds. According to AT&T tech, my older SIM card was provisioned for 3G/4G LTE service while the new one is provisioned for 4G LTE/5G. And possibly because 3G cellar service was terminated on my tower this summer caused the issue to surface. Hope this helps.