I’ve experienced same or similar with my AR300M in WISP repeater mode when it is in the same room as Comcast Xfinity WiFi signal it is repeating. When this happens, I observed the available SSIDs in the area from an unconnected laptop. AR300M was -33db, about 12 other SSIDs, including 3 or 4 hidden Xfinity signals, 3 private Xfinity signals and 2 public Xfinity signals(each related set with unique but sequential MAC addresses), and a neighbor’s refrigerator were -85 to -72db. If the AR300M SSID was visible, within a few seconds, my Xfinity SSID would disappear and the AR300M would lose internet connection. About a minute later, the Xfinity SSID would return and AR300M dropped out. Clients could connect directly to Xfinity gateway, except for the AR300M which remained unable to rejoin. The AR300M network page just perpetually showed “reconnecting”. Each time I manually disconnected the AR300M or rebooted the process would repeat, with the AR300M sometimes supplying internet for a short time, sometimes just going through a perpetual connect/disconnect cycle.
Moving the AR300M so that at least 1 wall was between it and the source WiFi eliminated all problems. I suspect some kind of constructive /destructive or multipath interference but do not have the means to confirm.

I have experienced similar thing when trying to connect to weak signal Xfinity APs while traveling using phones, laptops, or other wireless routers, so I doubt the issue is with the AR300M. It has rarely happened while trying to connect to other infrastructure WiFi that has multiple APs but a single SSID such as airports and hotels, but public xfinitywifi SSIDs have been the worst. Usually the SSID will stay up long enough to sign in and pull a small amount of data for a couple seconds then disappear for 20 seconds, come back up and make you sign in again. I do not like the fact that Xfinity Gateways broadcast so many different signals including multiple hidden ones that cannot be turned off. That could potentially be contributing, but I cannot confirm.

I do not know if the cause of these similar but different situations is the same. Hopefully this additional information will strike a chord with someone that will recognize what is actually happening.