C3n Before doing anything, I'd suggest resetting your mobile network settings to see if that helps. You can do that here while in the owner profile: Settings > System > Reset options > Reset Mobile Network Settings
You can also try different settings in Settings > Network & internet > SIMs > China Mobile. From there you can try different settings, like try toggling "allow 2G", or try changing the preferred network type.
I could be totally wrong about this, but I don't think Chinese networks are supported, so maybe the phone won't connect without making some changes. You might need to make some adjustments to your APN settings, like Michiel suggested. I'm sure someone at the China Mobile store would be able to tell/show you specific settings you can try out.
I found an old page (https://www.jianshu.com/p/8af304a15892 in Chinese) online with the APN settings for China's three carriers. If you can't speak Chinese, you can at least show them the page and ask them if the information on it is correct. If they're incorrect, at least the employees at the store would know what you're looking for. Or you can try them yourself to see if there's any improvement, but make sure you apply the China Mobile settings, not the other two carriers' settings.
I'd take screenshots of what you have before making changes so you can change them back manually, but you can always reset mobile network settings again in Settings as I said above.