Any information on model number and hardware version of the product? Different devices have different Specs.

You can try to log into the management page of the TL-MR6400 to setup the date settings function. https://static.tp-link.com/2018/201807/20180702/1910012415_TL-MR6400(EU)_3.0_UG_REV3.0.0%200629.pdf...

It looks like your network diagram issue, like the router which provide the wireless network is not in the same network with the server PC, because from your describe, there is no device which can...

If you have not tried use the driver on the official website, you can try to uninstall the current driver and reinstall the driver on the official website to have a check....

You can contact TP-LINK Technical hotline/chat service to confirm whether your device is under warranty. I think different country have different policy.

Hi, because you disable the DHCP function on the VR400, so the VR400's ip adrdess must in the same subnet mask with your main router, then the clients which connected to your main router/VR400 can...