WIFI problem

I have the exact same problem. Even better, when my watch attempts to reconnect to the phone after failing at WiFi it freezes the watch and forces me to hard reset it. I've got an open support ticket for it.

I have sent them an email but no one answer

When the F5+ was released, it had severe issues with the Wifi connectivity, but this was solved by software updates. The only router I found so far, which refuses to work with Garmin watches is a Vodafone Cable router, all others work perfectly. So there is no general issue and without more information, I can give only general advice:

Delete the settings and setup new.

Did you change the frequency? Garmin watches (and most other smartwatches) don't work with 5GHz

Any other changes (MAC settings, router update, etc....)?

Did you try other Wifi networks (e.g. a hotspot with your phone)?

J'ai change mon router en 2GHz et fait un reset en ca marche :) MERCI!!

De rien!

i have exactly same issue, trying hard reset now. Btw I tired on several wifi networks to rule out possibility of wifi issue

I found where was my problem. It looks like no matter what, the Garmin watch is initially always trying to use, 

one.one.one.one service for DNS (1.0.0.1). That has been blocked on few networks I've been trying it.

I found that using tcpdump, after allowing it all works now.

Anyway I believe it is a bug, as the initial connection uses hardcoded DNS server above, yet the queries after are going normally to any DNS server as offered by DHCP

Same problem here.