Low speed wifi when using wAP as accesspoint

Hi there. I bought the MikroTik wAP ac - Outdoor dual radio AP 802.11n/ac. Installed it upstairs to use it as wireless accesspoint because my router wifi isn’t strong enough to get upstairs. I’ve connected it with my Asus RT-AC1200G+ router by cat5e LAN-cable: from the ethernetport of the Mikrotik to a free LANport on the Asus.

I’ve read many topics in this forum to configure my Mikrotik. Tried several configurations (also did a reset with select “No default configuration”) but no stable one found. Finally I configured it by using the Mikrotik app on my Android phone. In the quickset menu runs the Mikrotik as “WISP AP”. Now the system is running stable but the WLAN speed is very low. On several devices I checked it by using the website fast.com. Maximum speed is about 20 mbps. When I use my laptop connected with the (Mikrotik) LAN cable to my router, I get maximum speed of my internet-provider-subscription: 250 mbps. I also get this maximum speed from both the wireless signal and LAN on my router. So I conclude there is nothing wrong with my Asus router, LAN-cable between Asus router and Mikrotik wAP, and all my devices function well.

What am I doing wrong?

If necessary I will post some screenshots of my configuration