Thanks @AM-GL. Will give it a try.
Upgraded modem firmware to R03A04 - no joy. Then router firmware to beta ver 8.0 which stopped my EE SIM from connecting. Had to backout to ver 4.74. So back to square one. Wonder if Ive got a duff X3000.
@ Firesign. Can you say how you changed to USB? I checked the ISPReview site. Ran the AT commands but they locked out my X3000.
Hi All, I am having similar problems.
Had the GL-X3000 for a month or so and been working fine on 4G. I thought this was the location of the router. Tried external antenna and even moved the router outside with extension to test for 5G connection but nothing. Phone getting 5G and 100mbs right next to router but router refuses to connect to anything other than 4G LTE. Tried suggestions above and pasting that text into SSH, various firmware (including modem), including latest and beta, manual set up changes etc etc but just no luck. Next step is new SIM and ordered a Cudy P5 from Amazon just to try and compare. Any other suggestions welcome
Many thanks AM-GL 
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I put my new EE data only SIM in my new Puli AX (GL-XE3000) router and immediately came up against the "SIM Not Registered" message.
Your solution has got me up and running. 
https://fw.gl-inet.com/download/RM520GL-modem_firmware/RM520NGLAAR03A03M4G_01.202.01.202.zip
The documentation for upgrading the modem firmware doesn’t actually have this version linked. I’ll see if I can get them to update it.