The Fenix 5 plus had severe Wifi issues when it was launched. The problems were solved with several updates over the years.
I could not connect with my Vodafone Cable router, but since last week even that works. As this was the only router, which did not work with the F5+ (and with my sons Venue), it was very likely an issue of the router and I assume that an update of the router solved it, as there was no update of the watch.
In general the Garmin watches (and most other smartwatches) do not connect to 5GHz Wifi! Maybe this is helpful for those who still have issues.