I’ve reflashed OpenWRT on the Dual Gigabit Ethernet Carrier Board for Raspberry Pi (it really needs a name) to the latest version since it apparently ships with bad firmware. However I’ve found that it corrupts the filesystem of an attached USB Hard Drive. I got it hoping to use it as a NAS so this is kinda a big problem. It doesn’t seem to matter what filesystem is on the USB drive, as both NTFS and ext4 files and filesystems become corrupted. I’ve since been using the USB drive with my computer and it has performed flawlessly so the problem is definitely with the CM4 carrier board.
I experienced interface flapping on the router kit because that interface is on the USB bus, and that CM4 to carrier board connection is very sensitive to overheating. After adding a huge 2x4x6" oversized aluminum heatsink I had on hand, I’ve been able to keep the CPU temp down to ~35C and the USB bus seems to be stable there. Try improving thermals and keep track of the CPU temperature.
Update: my board appears to have died, and maybe taken the CM4 with it as a spare carrier board won’t power up with the CM4 seated.