Help me making a home network behind a router

i want to set up a network behind a router. It currently consists of 5 pi 4 (4 gig of ram) & 2 other pc's. The router is a sky Q router . Which must be used, all kit is within 2 meters of router & all have wireless connection. Some are wired as well, all machines run Linux (mostly Debian buster or ubuntu ) all systems already have ssh or xrdp already installed & enabled. the use of web-min where ever possible would be great. the router as 1 external dynamic ip. Each machine should be reachable via that ip, the router only as space for one dmz ip, as each will have a FQDN. These are all purchased from namecheap who as it own ddns system, but this should be checked for being correct. Not all FQDN have yet been assigned, & i should be shown how it all works so i can add and alter machines as required. (that is possibly the difficult bit i can be very dense ). no payment will be made till i am 100% satisfied.