I've now tried some other things as well,
Checked UPnP is active on the router, a Huawei R2561. (it is)
Created port forwarding list on the router just in case, based on information from https://portforward.com/dcs-world/, (image below, no effect)
Added Outbound Port Rule in Windows 10 for DCS.exe on all ports and protocols. (No effect)
Tethered my phone to the PC to use a totally different internet provider. This DID work and I can see all servers (although with horrendous ping)
I don't know if there is some other setting on the router I might be missing or if for some reason my provider is blocking DCS multiplayer servers. Although that last one seems unlikely, as I was able to by change get onto the Growling Sidewinder server in once instance using [Connect by IP]. Is there any way to check if they are blocking something? It's also weird as a while back I was able to see all DCS servers through the same internet provider and router so something must have changed.