I was told by Apple support that whether GPS or Cellular, your watch must be within bluetooth range, which is about 30 feet. However, this support document says that your watch can run independently on WiFi even if your phone is turned off. See: https://support.apple.com/guide/watch/use-apple-watch-without-its-paired-iphone-apd0443fb403/watchos
Per the document, you must have a WiFi-calling to make use of the watch without the phone on. For that to work, you must in turn have a WiFi-call compatible cell plan. (To check, see if you have an "Advanced Calling Features" menu under Settings>Cellular. This may not show up in your phone if you are using a prepaid iPhone and/or a prepaid plan.)
In my experience, the watch can still call over WiFi *without* a WiFi call compatible plan — but sending/receiving calls is more successful at greater distance if you turn the Bluetooth on your phone/watch OFF. This forces the watch to stay on WiFi, when otherwise it might report that the "Call has failed" due to being out of Bluetooth range.
For what it's worth, my experience with the Series 5 GPS watch is that Bluetooth was so unreliable, even when the phone and the watch are in the same room with your WiFi, that I exchanged my first Series 5 GPS thinking it was defective. The new one is the same way. I think I figured out the problem and that is that even though you may have a high-speed WiFi signal available, the watch "prefers" Bluetooth. I have found that when the watch reports "Call Failed" or gives the message "I"m sorry, I can't complete your request. Try again later" that the problem is typically related to the watch moving off the WiFi network and onto the Bluetooth. It doesn't automatically connect back up to the WiFi even when it should. I have confirmed this by looking at the Settings>WiFi on the watch and noticed that my Home network has moved down into the list of other networks in the area (neighbors). If I tap it, and the watch rejoins the network and I stop getting the "let me try that again" errors with Siri and the "Call failed" notices. Key to this approach is that if Bluetooth is Off on the watch, the success rate of sending/receiving calls at greater distance is greatly improved.
While this may seem like an overly-complicated answer, I wouldn't bring this up if I had a simpler experience. Having had two Series 5 GPS watches in a row that behaved unreliably while Bluetooth was ON suggested that the watch will attempt to remain connected via Bluetooth even when that connection is not ideal. Until Apple releases some kind of patch that allows it to handoff more cleanly between Bluetooth and WiFi, the standing Support advice is to stay within 30 feet of your phone. A better solution, IMHO, is to use Bluetooth sparingly and then turn it off if you want your calls/texts to work at greater distance from your phone. (It's still important to turn Bluetooth ON first thing in the morning and/or last thing at night to transfer your health data because that info isn't transmitted over WiFi and won't sync with your phone otherwise, but otherwise you might feel like the performance of the watch is more responsive/reliable if you force it to stay on WiFi, which in my experience means disabling Bluetooth.)
I know this was a long answer but when I had a lot of confusion over this same issue, it was crickets everywhere I posed. I hope I have helped you and other soon-to-be Series 5 watch owners out!
