Short story: edit /etc/dhcpcd.conf (man 5 dhcpcd.conf). Add the following block (usually there is a comment “Example static IP configuration”.
interface eth1 static ip_address=192.168.0.12/24 #static ip6_address=fd51:42f8:caae:d92e::ff/64 static routers=192.168.0.1 static domain_name_servers=192.168.0.1 184.108.40.206
Long story: when our routers boot, we default to the IP address of 192.168.0.1/24. When we need to do firmware upgrades using TFTP, uboot will also use 192.168.0.1/24. When I have multiple routers, all running with the default IP address that need to be upgraded, I cannot connect all the routers to my same Linux box. Confusion will reign.
My solution has been to use a Raspberry Pi connected to each router. The R-Pi main Ethernet port is connected to my cubicle LAN. A second USB Ethernet dongle connects the R-Pi to the router.
The R-Pi acts as a firewall client between the prototype firmware (which can and often does have whacky bugs in my code). The R-Pi is plugged into one of router’s LAN ports. (Usually there’s a single specific port hardwired in uboot that comes up.) The R-Pi runs the TFTP server. The R-Pi runs minicom to the router’s debug serial port. I also run a serial console to the R-Pi itself. If the router testing goes doolally and confuses the R-Pi TCP/IP stack, I can connect over serial (yay, out of band control!) and reboot the R-Pi.
Before the Great R-Pi Shortage during the pandemic, I purchased enough R-Pi that I can have an individual R-Pi for each development router. I find this to be a very reliable solution, except when I forget which R-Pi connects to which router.