If there is a serious bug on firmware 13.20.26 there would be an outcry here on the forum. No discussion of this problem on the 4 languages of the forum, rather an isolated problem because no known problem
Optionally check the configuration of your WiFi2EthernetWifi2 IOGear GWU637 gateway
Personally I tested on my gateway of another brand by introducing 3 devices all ended up with the IPs in DHCP without problem.
Cordialement Doremi