Yes at boot system to brings up the router but often the connection does not come up for various reasons., thus the monitoring daemon, systemctl does not restart the service when it goes down for whatever
reason either until I reboot.
My pyDECNET router THEARK runs on a Pi Zero W2. Normally all happy and peachy for months on end but from time to time it loses connection to its area router.
I have to ask... are using Wi-Fi or have you added an USB Ethernet adapter to your Pi Zero W2?
This could be your Wi-Fi dropping out. An examination of logging (using journalctl) would verify this.
Tony