I landed at BKK 4x last year, fast track is handed over by ground staff to J passengers when disembarking, don't forget to fill in your arrival/departure card in the plane before landing. I usually check in for my domestic flight within 30 minutes after hitting the gate.
If your plane gates at C concourse it might be a very long walk to immigration though (up to 1 km), D direct opposite IMM and E a 5 minute walk.

If your luggage get checked through (which I hope) you will follow transit and remain airside, if needed get your MH boarding pass printed.
2h05 is OK, watch out for MH gate closing but should not be a problem.