You can see the fault..
DHCP is not able to get an IP from the modem.. That is in the 3rd screen shot.. the IP is self generated.
When you connect the mac mini to the hua wei directly you immediately get a public IP on the express, not a private IP.. your address there is 182.55.105.235
But that is not the address the mini is getting,, so how is the express getting this address.. what has changed??
When you plug in the mini do you have to sign in to a web account to get an IP.. what is the method of authentication.. ??
The express is now in the wrong mode.. it has to be in router.. but the Mini has to be directly connected to handle the authentication. but I am guessing madly.
Please tell me the precise network layout with your later 3 screenshots.. and what IP the mini is getting..
And I am guessing the express and mini are both plugged into the huawei.