General Information

Just so that you know, malware bytes on a machine complained about malware on the imore site. But I think it's okay now.

Running Second Router in Router Mode: DD-WRT :

https://www.linuxquestions.org/questions/showthread.php?p=5840957#post5840957

The moderator approved my post. Plus there are more now, but there's nothing to respond to yet.

More VLAN Information:

https://en.wikipedia.org/wiki/Virtual_LAN
https://www.lifewire.com/virtual-local-area-network-817357

I'll repeat the answer here, for clarity.

You can't really get them to work in more than one subnet, without a repeater device. One such device, you can find on amazon, for $99. You can, have a DD-WRT router, act as an access point, more or less, which puts everything in the same subnet, and then airprinting will work.

As for my solution:

I want airprinting to work on clientrouter, so I made clientrouter act like an access point (a fancy one). I DON'T want airprinting to work on guestrouter right now, so I kept NAT and everything. However, to improve the way it functions, I punched a hole in the firewall, so that everything can get through NAT, and it works more or less like some other networks I have seen before.