Inserting my openWRT router between ISP's ONT and router *transparently*

[lleachii]

@StochasticKarma

I don't think configuring a router to forward a given set of ports to one of its LAN ports while keeping all the other ones for itself requires magic

This is a simple port forward/DMZ then - see example here:

Guide to set up DMZ via LUCI

To those who may read this in the future, this is the rule to make a LAN device DMZ. Make this the last rule in the Port Forwards, at the bottom of /etc/config/network - config redirect option target 'DNAT' option src 'wan' option dest 'lan' option proto 'tcp udp' option src_dport '1-65535' option dest_ip '192.168.xxx.xxx' option dest_port '1-65535' option name 'DMZ'

FYI, this will not do (and isn't) what you described in Post No. 1 though.