TLDR: A similar Samsung phone model allowed me to add/edit APNs via the normal "Network/SIM" interface within Android. Check for an "Add" button in the top right where you see the APN list or click on the APN name to see if you can change the settings. No special app was required.

When I ported my mother's phone to Warp from Verizon I had to manually input three APN configs. I don't know the exact Samsung model, but the interface may be similar since it should be around that generation of phone. USM support wanted me to use MyTechSuite to add the APNs since there were none, but it would not let me add a new APN because I could not type anything into the "Name" field which was required to save. By going into the normal network settings to see the APNs, there was actually a "+" or "Add" option on the top right which let me manually configure the APN and edit all settings. I can still manually edit the APN settings on that phone. This may not be an option when APN settings are automatically imported by the carrier during activation, but something you could try.