GL-B1300 - Wi-Fi is slow

Hello there,
I have a GL-B1300 and since sometime the WiFi is much slower that the wired connection, even if the connected device is very close to the router.
For example, the wired speed is about 100 mbps and goes to about 50 mbps with WiFi. Furthermore, the WiFi speed is also a bit unstable with big variations during the Speedtest.
Current firmware is 3.103.

Thanks in advance for your help!

Are you using cable to feed Internet to the WAN port? Or using repeater?

When using repeater, wifi is always slow.

Also check if you are using 5G wifi. 2.4G wifi 50Mbps is normal. 5G wifi should be much faster.

Yes, I’m using the cable to feed internet into the router.
With 5 G works a bit better but the speed is unstable and the range is very small

Is your Ethernet cable only 100Mbps?

Yes, it’s the max speed