Router

This usually means there is some problem with either the modem-router (I had software lockups on these things, desync issues etc) or the uplink.

Which model of modem-router is this? Did your ISP (Internet Service Provider) provide it to you? If so, call your ISP, have them fix it.

Many times, your ISP tells you to reboot your router, because that gets rid of any software lockup or synchronisation issues. However, this is a workaround and not a fix. You can look into the log entries of your modem-router to find out what your router thinks the problem is.

Usually, these devices have a web interface for that. You can often find the web interface if you look at your IP address (Linux: ip a, Windows: ipconfig) and substitute the last number with a 1. Your local IPv4 either starts with 10. or with 192.. So if your IP is 192.168.1.102, the router might be reachable at 192.168.1.1. The default private IPv4 network is often also documented in your device's manual.

Sometimes there's also a sticker on the bottom of your modem-router which tells you the default IPv4. Most of the time a sticker at least tells you the make and model number.