Catalysis of the reaction: S-adenosyl-L-methionine + tRNA containing N6-threonylcarbamoyladenosine at position 37 = S-adenosyl-L-homocysteine + tRNA containing N6-methylthreonylcarbamoyladenosine at position 37.
tRNA (L-threonylcarbamoyladenosine37-C2-) methyltransferase activity