Wireless Joint

I have a T-Mobile business tablet plan. The SIM was activated in an iPad and works as expected. I want to swap the SIM to a GL-iNet AP1300 but everything I've tried has resulted in a "SIM cannot be registered" error.

Set APN to fast.t-mobile.com
Enabled IPv6 in GL-iNet default admin interface
Spoofed IMEI to match iPad
Set TTL via GL-iNet default admin interface to 64,65,66
Used custom firewall rules to set TTL/HL to 64,65,66 using iptables and ip6tables commands
Flashed to latest OpenWRT firmware
Flashed to latest Rooter/Goldenorb firmware

Tweaked settings under the custom firmwares but nothing allowed the device to connect to the cellular network. This device definitely works with T-Mobile per specs. I also took a different SIM from a T-Mobile issued Inseego router and tried it in this GL-AP1300 (after a factory reset) and it connected with no problems, without any tweaks. Note that that SIM is linked to a business internet plan and not a tablet plan.

I also took the tablet SIM and popped it in a Samsung S8 phone and it connected.

There's lots of documentation online of people getting a tablet plan to work in a router such as this, usually by just modifying the IMEI (although not always necessary) and the TTL. I've tried everything I can think of but I'm not having any luck whatsoever. looking for any other suggestions.

It may just be a janky SIM card. I know you got it working in other devices, but it might just be a tad off. Order a new SIM for that line and try again.

Shoot us a screenshot of the error you are seeing, as well.

After trying everything imaginable, I turned focus to a hardware issue. I disassembled the router and removed the modem from the Mini PCIe slot. Cleaned the leads with some contact cleaner and reconnected. Low and behold the SIM registered. It's been running this morning with no issues so far.

lol, that sucks. Glad you found it!