Hi,
found a working solution now for KOBO Uncaged in the
KOreader Wiki,
the port's rules should be:
9090 tcp in+out allow
39001 udp in allow
44044 udp in allow
48123 udp in allow
54982 udp in allow
59678 udp in allow
This source says of the udp ports, that "At least one of these ports needs to be opened on the firewall".