Summary: Surface Laptop 2 Wifi upload is very slow (unusable) with Fritzbox 7390 as an IP client to the Fritzbox 7590 when other every other device uploads fine, and all devices including the Surface Laptop download at my full internet connection speed.
I have set up my Fritzbox 7390 as a wired client to my new Fritzbox 7590 so I can get WiFi to a part of my house where coverage isn't great. The 7390 access point on 5GHz works well with my phone (Moto G8 plus), laptop (new HP Probook), and older laptop (Toshiba ultrabook) but with my new Surface Laptop 2 upload upload speed is terrible at about 0.01Mbps.
All machines including the Surface Laptop get my full internet download speed of 100Mbps on the 7390. The Probook, old Toshiba laptop and Android phone get my full upload speed of 20Mbps on the 7390. The Surface Laptop 2 gets 0.01 or 0.02Mbps upload. All the laptops and the router are in my office, with the devices about 2m from the router and having clear line of sight.
5GHz was turned off for these tests. I turned it on and it showed the same behaviour, download fast, upload very slow.
Surface Laptop on 7390 Wifi
Surface Laptop on 7590 Wifi
Older Toshiba on 7390.
Here's a screenshot of most of the devices in the Fritzbox console
I've disabled 2.4G on the 7390. All APs are on channels 40 - 50.
Any ideas? Why is the newest, most expensive laptop doing so poorly on WiFi when all the other devices do fine?