This “Made in China” router using Linux 3.12.59 from 2010, is teaching the “expert user”. You have to get “expert user” if you want to switch off firewall. As of the time of writing, stable Linux kernel version is 5.5.6, longterm 5.4.22, earliest is 3.16.82. It is also using old versions of openssl and iptables.
Apparently the creators of this router never thought of any user employing Linux and iptables, or something similar.
Luckily, the router allows to forward a range of ports, thereby effectively bypassing the firewall.