I have a similar issue which has been going on for like 2 years now.
WIFI network will randomly stop working, but for DOWNLOAD only and only for some devices and in certain parts of the house.
So my wife's computer might suddenly lose internet access, but her phone will be fine.
The only noticeable difference I could find was that the computer was connecting to the 2.5ghz network and the phone to the 5ghz network.
When this occurs the wifi will be up, signal shows as full, everything appears to be working, but a speedtest will show download speed at almost 0, yet upload speed will be at 8Mb.
I check the traffic stats on the router, nothing is caning the bandwidth.
I even swapped out the router for a spare, same issues.