Hi, just wanted to say, I had the same issue with the GL-X3000 and the EE network, and I found a fix. It took hours and hours to find the right answer.
I needed to lock out the 5G SA bands, as per another post I found.
Go to Cellular Settings (where you change APN etc}, then expand the 'Advanced' options at the bottom, turn on "Enable Band Masking", Masking Mode "Open", Select all LTE bands, Select all NSA bands, but don't select any of the SA bands. Then click 'Apply'.
You can see a screenshot of it down the page here:
Hope that helps someone.