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Wrong guess, sorry: issue was back again today, even with IPv6 activated. And this time changing and reverting the band selection from the GUI was not enough, log were cycling and no IP address was assigned. I had to restart the Zyxel for good.

Finally, I'm afraid that only a watchdog to monitor an IP address and restart the connection, and then the whole Zyxel if if fails again, would be required to fix this 😥

I solved the problem.

For some reasons seems that with this SIM and/or provider a manual PLMN selection is required. 😕

@RobTheNetworkGuy I have several SIMs and noticed the bug may occure mainly with one of them (not 100% sure, but I don't remember having the issue with the other SIM). As one difference is this LTE provider is IPv4 by default, I asked this LTE provider to enable IPv6 and changed APN configuration to IPv4+IPv6. FYI my network behind (pfSense) is IPv4 only, so IPv6 is only on LTE side. For now, every 12 hours the Zyxel detects "RILCMD: ConnState[0] is not attach in RS_READY." and then restarts the whole connection process including DHCP (udhcpc) with a new IPv4 address… but yet no connection loss during the last 36 hours.

Do you have an IPv6 address assigned, and if not maybe you could give it a try? Again, only on LTE side: no need to enable IPv6 on your pfSense.

Addendum: 48h and yet no network loss, except a few seconds every 12h. It seems to be that Zyxel fails to handle the DHCP IPv4 new address, but detects it if IPv6 is enabled and restarts the connection properly. My 2 cents.