I have a decent router installed. It a TP-Link Archer VR-600. See here : - https://www.tp-link.com/au/products/details/cat-15_Archer-VR600.html .
Incidentally I went to my son's house today and his smart TV is hard wired via Ethernet cable and he was also having trouble with Netflix picture quality. It was a lot worse than my WiFi connected Humax box reception so I am wondering if it is a Netflix or ISP problem and that they are sharing band-with out among several properties. I believe it is called 'Contention ratio'.
Either way, having seen my son's Netflix picture quality I am now content.