Hi, from the information you mentioned, it should be the two routers not support NAT-T, if they are support support NAT-T, you can see packets goes to port 4500.

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.