Synology Wi-Fi low speed fix

I have a mash wi-fi setup of 4 routers and have very low speed on both 2.4 an 5Ghz network. Wired connection works in full speed. For test purposed I have disable all other networks and left just 1 5Ghz. Wi-Fi scan app shows no other networks around me. The speed is still low - up to 20mbps. My uplink connection is 256mbps. https://www.speedtest.net/result/12667095842