I have seen other posts and people talking about this exact problem but have not seen a solution.
Problem: Can join friend's shareplay when they host (not xfinity customer). When I try to host (xfinity customer), when anyone tries to join my session they are unable to connect.
Tried another xfnity customer hosting and got same result.
However both myself and the other xfinity customer are able to connect to a third person's shareplay when he hosts and he is NOT an xfinity customer. The non xfinity customer can not connect to my shareplay or the other xfinity customer shareplay.
Tried new router supplied by xfinity.
Tried disabling advanced security features on xfi.
Tried placing ps4 on DMZ.
Tried connecting PS4 both wired and wireless.
I have done some research on this issue and the culprit seems to be modems that use the Puma 6 Intel chipset. Arris has actually faced a class action lawsuit because of defects of the Puma 6. If you're still available, would be curious to hear if both your new modem and old modem were Puma 6 types. I have a Puma 6 type modem supplied by Comcast in January and all my shareplay problems began after getting it. My previous modem worked perfectly fine, really regret sending back that old one since I'm not sure if Comcast will send a non-Puma 6 type even if they were to supply a new modem. I'm looking into buying my own modem and seeing if that works since I use/used Shareplay often. In addition to complete shareplay hosting inaccessibility, streaming has been of uneven quality and the connection drops every so often.