RADIUS Issue (Windows NPS with Single DSL connection)

Hi all, encountering a strange problem at the moment.

A typical site for us has a leased line with DSL backup. Certain sites only have DSL while we wait for digging to be done.

On a DSL only site, I cannot get RADIUS to work with our Meraki WiFi. If I enter valid AD credentials, the request never seems to hit the server and the connection fails.

If I enter some random characters then the request connects to the server but fails due to the fact it’s not a valid account.

As above, this is only on a Single DSL connection. It works perfectly on a Leased Line. My initial thoughts were MTU but I’ve had this changed by the ISP to no avail.

Any feedback would be greatly appreciated!