Hi,
When using legacy firmware version, you should be able to see your APN in Network -> Mobile -> General -> SIM1 (under mobile configuration).
When auto APN is enabled, the line below states which APN is provided.
Otherwise, the custom APN can be used.

On the latest firmware, you can find APN on the interface page. Network -> Interfaces -> General -> Mobile Interface window (mob1s1a1 / mob1s2a1)
Kind Regards,
Andzej