Most VPN's should work but depending on the qualiry of the VPN and their Server Speeds it could cause buffering so if you see its causing issues try a different server location or turn off the VPN
If you are looking for a good quality VPN we suggest Epic VPN.
Search Google playstore or your Firestick app store for "Epic VPN"
Once downloaded go into the VPN app and turn on the VPN to a server in USA or Canada then go into the IPTV app and see how it plays.
Let me know what you see after turning on the VPN and if it improves things.
Epic VPN is the best VPN we have found for IPTV so if it works its definitely worth the subscription cost to buy it to get their premium servers ($9.49 per year) if you decide to use it going forward... you do not have to buy, you can use the free version but to get rid of the adds and get the premium servers its totally worth the small cost.