small home networking lan problems

[srs008]

okay i'm somewhere between a complete network novice, and at least functionally competent. with most of my pc experience actually in the xp era. it seems the newer the os, the harder it is to get it to do basic things.
early on file sharing was important., since win 10... not so
these days. our internet is pretty good (for the area). our gaming computers have plenty of storage. and lan cables direct to the router.

recently we had to change router. which has caused. well pretty much complete isolation.
after hours of chasing around internet nat guides, i finally got the "second" gaming machine to CONNECT to mine running a quake 3 server...
not to FIND it though. manual ip entry. that was a serious victory over the complete lockdown previous.

our network is fairly simple. a dsl-3900 router takes a fiber optic connection (right to the nbn box). and then splits the internet between a multimedia pc (ryzen 3400g) (wifi)
my gaming pc and my brothers machines. both connected via lan.
these two machines need to be able to access online game services, e.g xbox game pass. at the same time. and be able to host, or join the same lobby.
just as importantly, they need to see eachother for games that run in lan mode (i suspect our recent attempt to play craftopia was it falling back to lan mode, and failing to find the "server" immediately after, hence the moment the second player appeared, the loading beginning, and then freezing till it disconnected.)
another game i tried was streets of rogue. under the old router, the lan mode found a second pc without issue. and the same was true for joining the same online lobby.
nothing under the new one.
i think the best evidence of this being nat, falls to cod black ops II, pc 1 connects. all good.
pc 2 got stuck at the "update" part of the process. and was unable to login.
eventually, we pushed it through, and it still could not find a lobby.
"from the depths" also uses a co-ordination/facilitator master server. pc 1 can see a few lobbies. while pc 2 sees none.

the old router worked on cod and the games we tried BARRING "from the depths" (which has always been problematic), and craftopia (which was not on gamepass at the time)
by port forwarding the game ports to pc 2, and then setting pc 1 on the DMZ.
as i understand it, this should not have worked. but clearly. it did.
this does not work now.
every guide i see says basically says. get a new internet connection. like a fresh isp (on a new line), or a mobile dongle. or turn on nat (which say's on? i think.)
it also seems the instructions for my router don't quite match, maybe they removed some options in the latest version? or revoked a newer version?
since i can barely remember most things minutes later, you can guess my success from over a fortnight ago.


thankfully, the "multimedia" machine only needs to maintain video service access while we game, as it's mum's video device.

i know it's long, i went through three longer drafts. i'm on the wrong end of my sleep cycle. so i'm dropping for a few hours.