Try a good VPN a lot of them have free trials and see what happens.
As a test try throwing the hub into modem mode. In this mode you will need to use a wired connection or get another WiFi router connected.
With a plex server it can buffer sometimes if it's doing a lot of conversion to fit the screen size and resolution so you may like to have a quick look at your settings (plex.tv website and your player) and see if changing that a little may help. Normally selecting 720p is pretty safe.
I had issues with my seedbox (Feral) / plex box. Turns out that if I re-routed my Virgin IPv4 through the admin panel of my seedbox it solved problem. The issue maybe a route problem with Virgin overall since I've tested with Sky Fibre (80/20) and no buffering occurs without the re-route set, currently on the M100 plan with Virgin with re-route enabled.
I would not advise using a VPN to stream via Plex as this will just slow down even more, yes I tried this before I tried anything else and it was a no go.
Best bet would be is to find a Seedbox provider that offers Plex, and an option to re-route as this is the case with Virgin works 99.9% of the time. 🙂