DIAGNOSIS This is almost always a power issue, not a Wi-Fi issue. The Doorbell 4 needs a consistent 8-24V AC from your existing doorbell wiring or a compatible transformer. Voltage that dips under load will cause it to reboot and drop offline.
FIX First, check your power source. If you're using existing wiring, you need to check the voltage at the doorbell terminals with a multimeter while the doorbell is connected and trying to boot. It should read a steady 16-24V AC. If it's lower, or if you're using the plug-in adapter, ensure it's the official Ring model (the one with the white rectangular power block). Third-party or underpowered transformers are the #1 culprit.
IF THAT DIDN'T WORK If voltage is solid, the issue might be your router assigning a conflicting IP address. Go into your router's admin settings and set a static IP address for your Doorbell 4, or enable DHCP reservation for its MAC address (found in the Device Health section of the Ring app).
HEADS UP The "Quick Release" battery on the Doorbell 4 is only for backup and feature power; it doesn't run the device full-time. If you're relying on it charged via USB, the device will constantly struggle and go offline. It's designed to be hardwired.