edit: I think "Guest Network" @wittaj recommended would be what I would try.
Play dumb and get lucky option: hook up a second distinct network router
Check if your ISP allows you to connect multiple devices to your single internet connection (i.e. thinking a switch in front of two routers to accomplish your separation goal). I have never personally encountered one that would allow it, but maybe you get lucky.
If your ISP provides your modem and it has multiple ports but your current internet only uses one of them, you could call them and "play dumb" asking them to turn on an inactive port to hook up "just one more device" (of course you will be hooking up a router with numerous IOT devices but they shouldn't need to know that).
Double NAT
Another possible option is double-NAT your entire home network behind two routers with NAT (beware double NAT can cause some hard to troubleshoot problems):
WAN--------Router 1 (IOT) ------- Router 2 (HOME LAN)
If you reversed the routers and put Router 1 behind Router 2 then you aren't actually isolating IOT from HOME networks like
@wittaj mentioned above plus all the IOT devices would be double-NAT (depending on the device this could be a problem).
ASUS DMZ
I researched if ASUS DMZ might work, but everything I read is that if the DMZ hosts are on your primarily LAN subnet (IOT devices plugged into your network), then if they were compromised they could launch attacks against your regular devices. (so bad news).
The "Longshot" - Custom Firmware + special routing rule
I found an article that describes a simple way and a more complicated way (VLAN) to setup ASUS routers for LAN-side separation, which might prevent you from needing to buy additional equipment. Here:
LAN port isolation (port-based VLAN) on ASUS RT-AX88U with Asuswrt-Merlin 384.16
Without a working ASUS router available to test, I don't know if Merlin firmware would be required or something. He supposedly does what you want with this command:
Code:
# eth3 maps to LAN port 2 on AX88U
ebtables -A FORWARD -i eth3 -o br0 -j DROP
The best option if you can afford to do it
Probably a better option would be some different firewall device that supports exactly what you want (VLANs, OpenVPN etc), and then repurpose the ASUS router as just a wifi access point (not router mode). Depending when you bought it that's probably a sour pill to turn your high end ASUS router into a dumb AP.