I made a ticket about this issue, because the connection from my FP3+ to my Fritz!box 5530 router is abysmal on the 5Ghz band in Wifi5 mode.
On any channel (32-128) the download link is switching continuously from 120mbit to 7mbit or connection fails on the higher channel (64 and up)
I switched the wifi settings on my Fritz box from wifi6 or wifi5 to wifi4, and the problem is gone. However my other devices suffer from this low bandwidth ( from 700mbit down to 150mbit max).
On lineageOS this problem is gone btw.
Not heard from Fairphone yet.