I needed to check something in my router's web UI, which happens every once in a while. I was denied because of an expired SSL certificate!
I use Let's Encrypt to generate all of my SSL certificates. It hasn't failed to renew (which it has on occasion). I run the certbot on one server, which has all of the software on it, and then copy the necessary certificate files to the router, into the location as configured in the UI. All set up long ago, so a little remembering was involved.
I checked, and the current certificate on my server had been renewed. Has 54 days until it expires, while the one on the router claimed to have expired a week ago. I was able to SSH to the router and check, and it had the same cert files as the server, so the copy worked.
A quick internet search and I found the command to restart the Web UI on the router. I ran that on the router, and the right certificate was presented. I ran the command again, and it kicked me out of the session, back to the login screen. I added the restart to the certbot deploy script that copies the files, touched one of the cert files, and ran it again. The files correctly updated on the router file system, and the Web UI kicked me out again.
Thankfully I'd had to set up all of the SSH things before to get the copy to work. Always play with your routers!