The first step is usually to visit fast.com and speedtest.net to measure the bandwidth you can achieve with your connection.
Netflix recommends that you should have a service providing 25 Mbps for a good experience.
It's worthwhile also noting the unloaded and loaded latency.
I run a number of routers and find that sometimes my devices claim to have no internet connection when everything else is fine. Usually disconnecting and reconnecting is enough to clear this on laptops and TVs (e.g. turn of TV briefly and back on).
Sometimes a router may end up in a bad state and anything to trigger a limited restart should clear (e.g. a reboot in the interface or same change requiring the WiFi radios to be restarted).
