I wish i’d Seen this post earlier - I could have potentially saved you £119.
1) The Solo2 is ancient, slow and largely unsuited to IPTV, not as bad as the older ZGemma crap, but that mips processor is geriatric by modern standards. I say this as a long time owner of it an an S4K. I know why you have it and why you use it, but better options exist for considerably less money than you have just paid out for the router.
2) While a VPN isn’t required for IPTV, I know why you would choose to use one as it negates 3rd parties blocking the server(s) in question and seeing what you view. Most IPTV streams are under 8mbit, it’s unlikely if that’s what you are actually getting that buffering is due to the connection speed.
Solutions that work:
Drop the VPN encryption level used, PIA offer a much less secure but equally functional option from memory and it works well for what you need, not so much if your life depended on the privacy of your connection, but it doesn’t.
Run a docker/VM with the Privoxy Proxy. It needs a host machine to run on, but it’s easy then to just set a proxy on the box, use strong encryption and see full line speed with virtually zero overhead.
Running a VPN for the whole LAN on a router is less than ideal, especially if the connection drops and your router then lets traffic flow over an unencrypted connection.