I was reading TomatoUSB forum and saw a message regarding forwarding ports via UPnP. It seems is not allowed for ports below 1024. How both Merlin and Asus firmware handle this question?
Asus also limits the lowest port to 1024 by default. You can override that either by using a custom miniupnp config file (in Asuswrt-Merlin) or by entering an nvram value that will override it (applies to both Asuswrt and Asuswrt-Merlin):
nvram set upnp_min_port_int=1
nvram set upnp_min_port_ext=1
nvram commit