Issue of two Xi6-A

You mean two Xi6-As work only Comcast Gateway.
Using my own modem with one Xi6-A and cabled TV box. Is it another option?

Hi Izum. Thank you for visiting and posting on our Forum. Our Xi6 box requires an Xfinity Wireless Gateway. If you'd like help getting upgraded to our rental gateway, I can help with that order. Please reach out to me via private message and include your full name and service address so I can assist you. 

Thank you, ComcastJoe Tru,

If I replace one of Xi6A to cabled TV box, I can use my own modem?

Originally I had only internet service and flex by using my own modem and Xi6-A.  It worked well.  After I added TV service, my own modem and Xi6-A worked well too.  After I added additional Xi6-A, I have trouble.  I should add cabled TV box.

It shouldn't have been a problem. It might have been too much signal for the splitter to push to both cable boxes at once and sometimes the splitter can be the root cause of connection issues to multiple devices. If your modem falls within our compatibility guidelines, there should be no issues with your modem. Let's see if we can isolate the issue. Can you try the using the additional Xi6-A, and try connect it as the main cable box? If it boots up and works, that would show that the problem isn't the cable box but the connection itself.