Bad Internet

Hello everyone,
the internet is just not fun for me. Constant crashes, bad ping, and hardly any performance. I can't even watch a YouTube video in peace without changing the quality constantly or pausing completely.
I have the following structure:
Fritzbox 7360 on the ground floor with 100mbit line from o2. So far everything works.
TP-Link Archer A7 (https://productz.com/en/tp-link-archer-a7/p/pELgq) in WISP mode on the 1st floor connected to the Fritzbox via WiFi
On the 1st floor, I get a maximum of 16 Mbit through the TP-Link router and this also varies extremely. The upload is more in the single digits.
If I log into the Fritzbox with another device (e.g. mobile phone or laptop) right next to the TP-Link router, I get the full 100 Mbit through. So location can't be the problem. Apart from that, I've already tried other places and it's always the same result. Just like other speed test sites, always the same picture.
It also doesn't matter whether I connect to the TP-Link router via WiFi or cable.
Does anyone have an idea where the performance is lost?
Unfortunately, it is currently not possible to get the connection from below via cable. Should change soon but now I have to work that way.