Issues with compatibility

Has anybody else had issues with getting a phone that is compatible with Xfinity I've bought two brand new phones off of Amazon which both said they were unlocked for all carriers including Xfinity and I get them and I put in the MEI number they're not compatible. I live on disability so I am on a very fixed income and especially my mom just recently passed away and I'm trying to take care of the that way so I can't buy a phone from Xfinity I have a Samsung phone and I've gotten two brand new phones from Amazon the last 4 or 5 days and when they don't work I have to find a ride to take them in to send them back to Amazon wait for them to refund me the money and then buy a new one and wait for it to get here I am so frustrated I wish I wouldn't have gotten Xfinity at this point