@KTW0 
Check the security authentication mode from your Deco's SSID, is it using WPA3?

Try setting it to WPA2.

  @Omochao I'm already using WPA2 but the problem persist. 

  @KTW0 

Does that modem comes with 2 in 1function such as WiFi enabled?

Could it be your modem is also a router and is broadcasting WiFi SSID? might cause interference?

If not, I suggest you to reset the Deco X50 with pin hole and set it all up, but turn off the access point and plug out all the ethernet cables.

Once you are sure that Deco X50 has internet through WiFi, only set back the AP and ethernet hard wire to your PC.

You can also try to turn off Beamforming or Fast Roaming first to check if its causing interference as well.