Compass Direction

Hello guys,
Just mounted my APM 2.8 to my fixed wing aircraft however to save space I mounted it the roll 90 direction.
For some reason my compass reads the direction wrong it reads south when my plane faces north how do I get through this any idea?
And I have my arrow on APM facing towards the nose of airplane

If you had a non obsolete Flight Controller that orientation would be learned during compass calibration and it would be good to go. One of many, many reasons to not use the old APM.

Is it an external compass or are you trying to use the internal one?

I’m using an external compass the one that comes with gps module