Andre, Hello and welcome; There are many 'it depends' to this issue; it depends on which OpenVPN client in the router you chose to run provider's config/tunnel up on (Client 1, Client 2 and so forth; if you're streaming to a computer browser, your smart TV, to a dedicated box, such as an Apple TV, Roku or Amazon Fire device and if you use a switch and/or 2nd router; if the tunnel is running on the same CPU core that the router's FW/GUI is running on, it can impact the stability and performance of the tunnel and your streaming); it can depends on the VPN provider you contracted with as well as how much bandwidth your ISP has you provisioned for, the distance between your location and the VPN provider's server, and if you're in the country you're streaming from; how congested your ISP's node is or how many users are present on the VPN provider's service/server; if the company provided you with the correct config file, or if you generated the proper config exactly as your provider instructed, or there's a single parameter in error as generated, and any of many other factors, which at times can be quite frustrating. When you toss in all of the security issues which apply to your setup and system, there's not always an easy or cut and dry solution, unless you want to drop all of your streaming to WAN, and let your ISP's servers handle your video streaming. If that was the case, you wouldn't haven cared about using a VPN.
If you have a tech-wise and trustworthy friend who leaves nearby nearby who's very experienced with this specific router and who is willing to actually lay his eyes on your set up, that would be an easier solution, since video streaming over OpenVPN isn't something the streaming services will never discuss with you. Searching through this forum and other streaming video streaming forums will supply you with a wealth of knowledge, as it's never as simple as having this particular router, even if it's a great unit, and that's where all of the other questions lead in.
If you believe that your router, your VPN config and your computer/TV streaming device is set up correctly, then along with devoting a lot of time to boning up on everything you can learn about the subject, the first step would be to contact or file a support ticket with your VPN provider's support team and ask them to review your config, and include the symptoms and any relevant log files with errors, if they ask you for them. Good luck and hang in there, it takes a while to learn, and everyone starts at the beginning.
