I have an UMIDIGI A7 Pro but I imagine other versions have the same internals re: the APN.
If the APN isn't automatically set up by default, to view your APN set up, or to change it, do the following:
From your Settings > Network > Internet & Mobile Network > Advanced Settings > Network
at this point you can choose to > Automatically select network (in my case PM)
or you can choose to > Access Point Names (in my case there are 3 additional choices + 2 options for further information)
Hope this helps. o/