No internet connection after changing LAN IP

[Fardenco]

I just installed OPNsense on a Proxmox VM to try it out.
My goal is to have OPNsense between my ISP router and the rest of my network.
Out of the box everything seemed to work fine, but to make things easier for me, I wanted to change the IP address of the LAN interface from the default 192.168.1.1 to 192.168.1.254, because this is the IP of my ISP router and I thought that it would allow me to connect all my devices to the LAN without having to change the configuration of the ones with static IP because the gateway address would be the same as before.
But after doing that, I can't access internet anymore from the LAN side. I can access the OPNsense web UI, I can ping google.fr from opnsense, but from a computer on the LAN side I can't even ping 8.8.8.8.
I'm new to this so I might be missing something obvious, but I've been looking for it four hours and I can't figure it out.
Do you have any idea of what could cause this issue ?
Thank you

Edit : I forgot to add that I made sure to renew the IP settings from the client to get new informations from the DHCP, the gateway IP has been updated as expected but that did not help.

[meyergru]

Yes. First: You have not given any details about the real network topology, so all of what follows is pure speculation. Experience shows that people who come in here and make their first post have little to no network knowledge, so here goes:

Router-behind-router scenarios will not work right out of the box, because you have to take into account that WAN and LAN of OpnSense must have different subnets in order to work, say 192.168.0.1/24 and 192.168.1.1/24. If 192.168.0.1/24 is your WAN, then your router must also know where to route packets destined for 192.168.1.1/24, but usually it does not.

If you instead only have a LAN interface for OpnSense, then it cannot do anything to protect you, because traffic in your LAN does not even pass OpnSense.

Also, when you change the main gateway's IP on your LAN, then any device that is configured statically will not use it as a gateway any more, plus devices that get their IPs via DHCP will still use the old gateway until their lease has expired. This also relies on having changed the gateway IP in the DHCP service as well. (you just added that)

Also, installing OpnSense on Proxmox has its own pitfalls.

So, yes, we probably have many ideas of what could have gone wrong, but your report lacks details. See this for a start.

[Fardenco]

Ok so I guess the main issue is that I didn't use a distinct subnet for WAN and LAN, and the fact that it did even worked to begin with is only due to luck.
I'll change the subnet and keep you updated.
Thank you for your help.

[Fardenco]

That was indeed the issue.
As you pointed out I still have a lot to learn !
Thank you for your help