Modem trouble

So I have bought 2 different modems to keep from paying the rental fees from Comcast, the first was a Netgear 16x4 and the second was an ARIS SB6183. Both of these units showed locked and ready to go both on xfinity side as well as my status lights. Everything said I should have internet. But I did not ! after running all the tests both with Comcast and again with the manufacture. I still could not get internet. I ended up having to put the rental unit back on my account because that’s the only one that would connect. It’s also way overkill I could care less about the phone and TV stuff . I already have a good router can someone shed any light on this? Has anyone else had this happen?