There is already some old locked post similar to this question, but not equal.
The router is 192.168.1.1/24
I added 192.168.1.254/24 to my "LAN" bridge to the router.
I modified DHCP server data to update clients over this new gateway address.
Probably in the LAN, there are legacy devices with static IP configuration continuing to use the old Gateway IP.
How could I know the IP / Mac address of those devices using the old gateway IP (torch, connection tracking, Nat rule with logging) ?