As a 91B how do I go on patrols and get extra combat training?

You can forget that as a 91B. In country you might be assigned convoy escort or maybe become part of a security team. Your primary mission is to keep equipment rolling so others can do their mission.

Better question to ask:
How do you, a 91B, get more 91B training to be a better 91B, which is the purpose to your existence in the military. No one needs you to know how to kick in doors. They need you to be a good 91B.
When vehicles start getting deadlined for electrical system issues, "but I know how to do a patrol" is going to be the most meaningless set of words to ever leave your mouth.

I was a Sailor, and an officer at that... pretty much the LAST guy one would expect to be convoying though the KG Pass behind a 240G... yet there I was, for nearly a year. Some things come down to fate... a lot comes down to choices. In the Navy, we say, "choose your rate, choose your fate". I'm sure if they let everyone have exactly the MOS they wanted, most people would want to be some kind of Infantry... the Army does a lot more than Infantry. If that's your passion, I'd suggest that you seek every opportunity to get into a combat arms MOS and excel there-it's not easy, but certainly "easier" for you than many. I knew a guy who gave up his commission in the Guard to get the MOS he wanted on AD as an enlisted man. I've heard of people going back through Basic after having earned significant rank during prior enlistments. Where there's a will... there's often a way.