S4 apple watch not pairing

Hello all,

I know that my phone and watch aren't exactly new (XS Max and S4) but I was really hoping that I would be able to get across the line and have them work without problem until I could upgrade when the new options are available in a few weeks.

Unfortunately, my watch decided to not communicate with my phone yesterday, and I feel that I have tried everything. Both have been switched on and off multiple times, both charged, network settings reset on the phone. I even completely deleted/reset the watch, and it no longer appears on find my iPhone and when I switch it on it looks like a new watch trying to pair, weird fuzzy orb animation and all.

But, all I get, quite quickly, is a message telling me: "Make sure this watch is powered on and is not paired with another iPhone" The speed with which this shows (and has many times by now) suggests to me that this is not just a timing out, but rather some other problem, but I feel that I have exhausted my options.

Any ideas? Please?