I’m writing here again because I haven’t received an answer yet! Is it not possible to deactivate certain countries (Switzerland) on the x3000? But in a simple way, without programming? Switzerland is not included in my O2 EU roaming and will be expensive!
Since it is only a GSM modem, as is also installed in cell phones, I would not assume that you can deactivate individual countries - this is also not possible on the cell phone.
But on my cell phone I can search and select networks manually!
I’m not 100% sure, but I think this should also work on the X3000. At least there are carrier profiles: Cellular - GL.iNet Router Docs 4
Did you try disable Data Roaming? Previous firmware has issue on this feature, please upgrade the firmware and try to disable it.
Please read carefully, I need data roaming, I can’t disable it! It’s about if I e.g. I’m in Italy, but at the Swiss border, the router dials into the Swiss network and that’s not included in EU roaming in Germany (home country)?
I wouldn’t think you would be able to disable connection based on a region or cellular provider. This would definitely be somtthing under the advanced settings or via a command line to adjust, if even possible to do at all.
Maybe a support employee has something to say? In any case, it would be nice, as many people here in Germany feel the same way, as Switzerland is not part of the EU and it’s stupid if you go on holiday in the border area, for example!
As a German, I can say: Just book the expensive Telekom tariff, which also includes Switzerland. 
Joking aside, the problem won’t be so easy to solve. Even on the cell phone, this can only be solved temporarily: As soon as the selected network has no connection for a short time, the cell phone jumps to another one - unless you deactivate the automatic network search completely, which then becomes inconvenient.
Of course, as described in my link above, you have the option of passing manual AT commands to the modem. You can also use this to permanently select a cell phone network.
More information about AT commands:
I am not sure but I would say that your question should be answered with the carrier profile: Cellular - GL.iNet Router Docs 4
Ein deutsches Forum wäre wirklich hilfreich für gl-inet Router, da würde man sich auch leichter tun mit dem schreiben 