"openVPN client I use keeps showing the message: "Server poll timeout, trying next remote entry..." Any atempt to access through a web browser using :80 or :5000 shows the message: "The connection...

the ddns client is registering the WAN ip address (or the first public ip address). check your real public ip address -> www.whatismyip.com

but after some time the x20 at the study room will switch the connection to ethernet ? The wifi speed is around 250 Mbits - with a mobile phone or with a notebook ? What's the type of the wifi card...

traceroute the 192.229.133.143 and the 192.124.249.118, check the path. you've changed the deco lan network to 192.168.0.x ?

the power is answer for @Irvin_Wells

@Yash2206 https://omada.tplinkcloud.com/login "forgot password?" -> click :-)