Is there any way to enlist into AGR as prior service with a break in service?

AGR is highly competitive hiring process. These are generally always reserved to current Guard/Reserve members in the State of the job posting. Like mentioned above, these positions are usually limited to certain MOS'. The first step is to be a member of the Guard/Reserves and look for opportunities to apply. You can increase your chances by trying to get a needed MOS.
I was selected for AGR on my 6th application over 2 years. Not an easy task but doable if your sharp and develop a reputation of hard work, integrity and a go-get-it attitude.

The National Guard in most states have online job postings. MOSs are limited for AGR. It helps to be in the Guard before applying. If you cannot find anything online a TXARNG recruiter could probably point you in the right direction.

You might wanna talk to a Guard recruiter. They usually have all of the low down on those AGR slots. Most of the time, I've heard that moving into those slots is usually reserved for people transitioning from Active or Guard into the slot. I don't know how easy it is to get into one from the outside. Good luck though.