Also I am being permissive with my answer because every carrier uses a different APN (Access Point Names in SIM menu). Since you are using a small carrier within a small OS, not many people can reproduce your setup and provide a definitive answer on what your settings should exactly be. So, it will probably take a bit of debugging and playing around with different settings before getting MMS working. I honestly doubt their customer service will even know what Graphene OS is. My guess is that Tello ships a working APN for IOS and the Android market is not large enough to deal with their problems. Tello used to run on Sprint before the merger and they might not have fine tuned their settings after joining T-Mobile.