You should be riding a large with short legs and long torso at 5' 11"
MTBs have a high Bottom Bracket for clearing rocks. If you are just riding easy trails look for a frame that has a lower bottom bracket, hence your saddle will be lower, easy to mount.
And I would say with 28 inch stand over your legs might be too short for a 29r.
Take bike back.
Look for 27.5 trail bike