I have an RT-AC68U and have never found a solution as all devices always get a network address within the main subnet. This even happens when using Guest 2 and 3 which others claim has resolved their issue.
IMO, the fundamental flaw w/ ASUS when it comes to the guest networks is having them share the private network. Now isolation can only be managed at the ethernet (layer 2) level using ebtables, which is often problematic. And if you dump ebtables w/ guests enabled and intranet access denied, you'll see the problem.
Here's what you get w/ guest #2 (2.4GHz) enabled and intranet access denied.
Code:
[email protected]:/tmp/home/root# ebtables -t broute -L
Bridge table: broute
Bridge chain: BROUTING, entries: 5, policy: ACCEPT
-p IPv4 -i wl0.2 --ip-dst 192.168.1.1 --ip-proto icmp -j ACCEPT
-p IPv4 -i wl0.2 --ip-dst 192.168.1.0/24 --ip-proto icmp -j DROP
-p IPv4 -i wl0.2 --ip-dst 192.168.1.0/24 --ip-proto icmp -j DROP
-p IPv4 -i wl0.2 --ip-dst 192.168.1.0/24 --ip-proto tcp -j DROP
-p IPv4 -i wl0.2 --ip-dst 192.168.1.0/24 --ip-proto tcp -j DROP
By default, it only blocks ICMP and TCP (and specifically for the private IP network, e.g., 192.168.1.0/24). *ALL* other access is permissible, including UDP, and any other IP networks that may be reachable from the private network (e.g., an OpenVPN client and its remote network). That's why you can't isolate guests from/to the VPNs w/o a third-party solution like YazFi. The fact YazFi even exists is a tacit admission that guests networks are fundamentally flawed.
This is why I do NOT use guest networks on ASUS firmware. You're far better off to use a separate router for such purposes (ideally w/ third-party firmware so you can manage its IP firewall), daisy-chained behind the ASUS router. Now the guest network is on its own ethernet segment and isolated at the IP level using an IP firewall. And as long as configured properly, will provide far better results.
Or else you have to try to implement VLANs as described other threads. Personally, I find using additional hardware easier for most folks. It's far less technically challenging.
Anyway, this issue of guest networks has been beaten to death, so it's not like it's unknown to vets around here. But just something newbs should be aware of.
JMTC