Learjet 45 Autpilot altitude hold but keeps losing altitude

Hope someone can help .I'm flying the default Learjet 45.

I set my altitude for example at 5000ft using the ASEL key

I engage the autopilot pressing the AP key then I Press the ALT key? But after a while it just loses altitude and crashes?

What am I doing wrong?

Also if I was to use the GPS , I select the GPS button next to the NAV Button   do i also need to switch the dial to FMS? The dial which has FMS, ADF1, NAV1 and off ?

What does the BNK button do? Is it for bank limits?