Catalysis of the reaction: a 5'-end (N2,N2,N7-trimethyl 5'-triphosphoguanosine)-ribonucleoside in mRNA + H2O = a 5'-end diphospho-ribonucleoside in mRNA + 2 H+ + N7-methyl-GMP. Can also use (N7-methyl 5'-triphosphoguanosine)-ribonucleoside in mRNA as a substrate.
m(7)GpppX diphosphatase activity