Selfhosted

Couple of things:

First, the subnet router for your wireless network is not 192.168.1.1. Given that the subnet mask is /24 and the subnet is 192.168.86.0, I’d guess that the subnet router for the wireless network is 192.168.86.1. Of course, you’ll need to verify that within your OpnSense configuration.

Second, by creating the two networks on OpnSense, each one likely already has a ‘default route’. On a Linux command line, the would be a destination of 0.0.0.0 with a gateway of 192.168.x.1. This means anything not meant for the local subnet (192.168.x.0) will gets passed to the subnet router.

Third, the firewall on the OpnSense router has to allow the traffic between subnets. This is likely your sticking point. You’ll need to visit the firewall admin area of OpnSense and configure each subnet to be able to pass traffic to/from the other. I’m a pfSense user, so I don’t know the exact steps in OpnSense. But these general steps should still apply.

Well in Interfaces -> LAN go to where you set your static IP for opnsense and change that to 192.168.1.1/16. That should get you running. But the google device would probably need to be told that it is 192.168.866.0/16 as well so it can see the 192.168.1.0 subnet.

However it would probably be better to disable DHCP on the google device, but I don't know anything about them. (I read that on some you cannot disable it, so set the DHCP pool to 1 and then assign that IP to some mac address. Essentially stopping the google device from handing out that address).

If this is what you need to do, then on opnsense set up your DHCP pool to say 192.168.1.100-250. Then set the google device pool to 192.168.1.251-251 and then set a static lease in the google 192.168.251 to MAC: de:ad:be:ef:ca:fe.

(That wifi sounds like a shit device - maybe consider a tp-link or something more configurable)

Are you using VLANs on your switch? Are you using the LAN or WAN port on the google device? As others have said, those two subnets do not overlap using /24 (255.255.255.0) so you would either need to use something like 192.168.0.0/17 that would cover both 192.168.1.0/24 and 192.168.86.0/24 but that is way overkill for most networks (192.168.0.0 - 192.168.127.255, 32766 hosts).

If you are having trouble understanding subnetting (or are like me and have a brain that refuses to learn any tricks to do it in your head) I highly recommend this really simple subnet calculator as it is very easy to see how you can divide subnets down from the RFC 1918 supernet (192.168.0.0/16) by clicking on "Divide" on the right side. In fact, that's pretty much the only subnet tool I use anymore, super quick and easy.

It might be easier to just disable DHCP on the google side (or configure it as a DHCP relay if you can) and just use one subnet from OPNSense.

If you are not able to disable DHCP on the google side then I would set up a VLAN for the google wifi device and then create a VLAN interface in the 192.168.86.0/24 subnet that DHCP won't use (like 192.168.86.2) and configure DHCP to use that for the gateway. This will then allow you to route between your two networks internally and to the internet (firewall permitting, obviously). If your switch does not allow VLANs then you could use another physical interface on the firewall and connect that to the LAN port on the google wifi device and do the same thing for the same result.

I hope all of that makes sense, please do ask for clarification if not, I do this kind of stuff every day and love teaching it so fire away.

This will fuck your plans but you should look at internal ipv6 routing, it's confusing at first but for situations exactly like this it's a gamechanger.