Catalyzes the breakage of the C-P bond in alpha-D-ribose 1-methylphosphonate 5-phosphate (PRPn) forming alpha-D-ribose 1,2-cyclic phosphate 5-phosphate (PRcP). AH2 + alpha-D-ribose 1-methylphosphonate 5-phosphate + S-adenosyl-L-methionine = 5'-deoxyadenosine + A + alpha-D-ribose 1,2-cyclic phosphate 5-phosphate + H(+) + L-methionine + methane Binds 1 [4Fe-4S] cluster per subunit. Belongs to the PhnJ family.