Frankenstein made a contract with Rai right?
When someone makes a contract with a Noble he gets stronger like the 12th elder.
But we haven´t seen anything like that from Frankenstein so far.
He is as strong as he was bevor he made the contract with rai (maybe a little stronger but that probably because of training over the 600 years).
Maybe we will see something in the future?
Most likely Frankenstein doesn't draw his powers from contract with Rai as that might reduce Rai's lifespan. Personally I think him not using contract's power is a way of showing that Franky didn't entered contract for the sake of power. A respectable way of showing his loyalty & devotion