Hello @danedwards_ee when no APN is specified ModemManager will retrieve an APN automatically.
Different APNs that are provided by the network usually correspond to different ipv4/ipv6 combinations - like one enabled with ipv4, but not ipv6, or one with both ipv4 and ipv6 enabled.So depending on whether ipv6 is enabled in the NetworkManager connection profile or not enabled a different APN could be picked up automatically.
If you still cannot get connectivity with ModemManager, but get one with a Teltonika gateway, could you please confirm the ModemManager version do they use? do you think you can share ModemManager logs? I also think it might be interesting to connect with ModemManager maintainers.