Apple watch 4 crashes home wifi

Tell us what you do and what you see with your eyes, not how you interpret it. You can connect the watch by several different methods. Router crashing can be anything from a software lockup to a full uncommanded reboot, or even failing and needing the software to be reloaded.

I find it it suspicious that you have problems with two different Apple devices. Details of the hardware for Apple and the router might hold some clues, and software versions for the watch and iPhone could also help.

That sounds more like something conflicting. If you let it go wrong then turn off WiFi on the watch (Control Center) does the iPhone recover its WiFi connection? Are there any other devices on WiFi like a computer? How do they behave? (is the watch only interfering with the iPhone, or all connected devices)

Your router probably has a log file, does this provide any clues after a lockup?

Are you saying the Watch crashes your Wi-fi? Router