The issue with Comcast modems usually stems from the modem locking on the first MAC it sees. It will then ignore any other traffic from any other MAC address. In the past there was a bug (see: Surf soho mk3 never finishes connecting to cable modem if modem reboots) that was resolved that leaked certain LAN ARP packets onto the WAN port under certain conditions. That resulted in the modem locking onto a MAC address that was not the one of the router WAN port and thus it was ignoring the WAN port talking to it later on. This caused the peplink router to stay in the “connecting…” phase forever. Even though the bug was resolved, I still see some LAN packets leaking to the WAN during the router boot up (Balance 20x, and SOHO) on some rare occasions, causing that connecting bug to happen.
So, in your case, I suspect the MAC cloning feature activates at some point during the boot. So it is possible based on timing, that in some cases, the modem sees the true MAC of the WAN port first, and then when cloning enables itself, the modem refuses to talk to the router which now has a different MAC. Removing cloning solves this case as you saw, because the router does not change MAC address during the boot sequence.
In general, the solution when this happens is just to restart the modem.
Hope this helps.
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