I spent a lot of time on this and try all, the methods mentioned (changing language and country, hard resetting the watch, the phone the network, using 2.4 Ghz wi-fi, logging off and on iCloud on my phone, updating in low battery mode, airplane on of and leaving the watch on charger for a night, using spare clean iPhone 6Plus for pairing, and many else I do not even remember now). Finally I went into my phone's current wi-fi settings and changed the DNS (settings ->wi-fi ->currently used wi-fi name scrolled down to DNS and changed automatic to manual and set it to cloudflare's 1.1.1.1 and 1.0.0.1) after saving the changes and restarting the pairing process again the phone immediately found an update and started downloading.
Hope it helps some of you
SIELU
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