Really? Where? (I know there are Senator lounges and Business lounges.)

I do not believe that there are any Star Alliance carrier lounges in FRA other than those operated by UA and LH? (Definitely no SQ lounge in FRA.)

I think that the OP is flying in on UA in F? But onwards on LH in C possibly?

Given that, I think the OP could get into the UA RCC, including the F section, and any LH SEN or BUS lounge?

I prefer the LH SEN lounge on the right as you walk towards the B41-48 pier. (My SQ or TG flights always seem to depart from this pier.) This SEN lounge has some very nice shower rooms, although several other SEN and BUS lounges have showers as well.