Slow wireless download less than 50

Apple tablets(2)and apple phone get good wireless.  Android tablets(2), android phone firesticks(2) get download speeds of less than 50.  Xbox wired is good.  All speed tests done within 2 feet of xfinity router.  Even your Xbox TV box is slow like the firesticks. 

Tried reset of router.many times.have 1.2 gig plan. 

Have discussed with assistance but no solution.  That was over a week ago.