You can definitely use the lounge if you are ticketed in Business, regardless if you used miles to book.

The lounge has no outstanding amenities, it has some sketchy looking hot food and couches for seating, that's all. No luggage storage. I made the mistake of asking to shower here, thinking why not since I had a longish layover and the lounge was deserted so I figured I could take my time - never again - and I think the attendant was also quite embarrassed about it because she was sheepish as she opened the single shower room they had... I wish I had taken photos. Objectively, it is a teeny tiny, dimly lit, damp smelling shower room, the water drains slowly, the fixtures are rusted, and in general doesn't look clean at all. No shelves or hooks to hang anything, the slow drainage will seep towards your handcarry bags if you leave them on the floor, which you must because there is nowhere else to put them. Afterwards, I felt like I needed a shower to wash off the experience. Avoid, avoid, unless you are desperate.

If FRA is not your original dest, you should shower there.