I have the GL-X3000 Spitz AX running 4.3.5 stable with a single T-Mobile 5G hotspot data SIM. The system boots fine with default settings. However, with IPv6 NAT6 enabled, during power on boot the cellular connection tries to start up but then goes down. I can log into the Admin Panel and bring up the connection manually.
I noticed the initial connection might be 5G SA. As an experiment I changed the profile to block all 5G SA channels. Now with IPv6 NAT6 enabled the power on boot completes with a 5G NSA connection and test-ipv6.com gives a 10/10 test result.
With regard to this issue, I tried the following beta image and was able to successfully power on boot into a T-Mobile 5G SA connection with IPv6 NAT6 enabled.
One thing I noticed with this beta version: When using the admin internet panel, manually switching to another T-Mobile profile resulted in an operation timeout pop up message, but the operation would complete in the background a short while later.