I have the same issue. I just replaced my M1XEP with a C1M1 about two months ago. Twice it has failed to connect and required the power to be reset. Once the power has been reset it works fine.
I have my firewall setup to log everything coming from the C1M1. If it is locked up or not every two minutes it was sending a UDP request on port 53 which I presume was a DNS query. Once it was rebooted I can see the requests to external servers on port 8888, 8899, 3066, etc. which should be the ones connecting to Telgard.
When the system locked up it still appeared to be sending the DNS queries, but not the queries to Telgard. I could also not connect to it using EkRP (local network and C1M1 proxy), but I didn’t try mini USB.
I thought about an automation rule that would flip a relay to reset power if there is an ethernet trouble. However, this is pretty annoying considering the C1M1 is supposed to be an upgrade from the M1XEP and separate Uplink module that was a part of the cellular sunset.
Is anyone else having this issue? Any suggestions?