restricting udp would be a bit odd - plus if it’s a basic BT router can’t imagine you can do that much to it ?
I do think there is something odd/unusual in any firewalls you have on the PCs that block traffic but on only on the same subnet ? - I know you are using Kaspersky but try disabling the windows defender firewall too ?
arguably this the problem you are actually trying to solve is the lack of sync when you are using over the internet so usual checks are for this are:
- are you wired both ends
- what buffer settings etc
- increase “remote delay”
- have you tried with a totally blank/new project
-update to latest version on connectvst.com