[Setup]

ER707-M2 gateway connected to my ISP (gigabit fiber).

SG3218XP-M2 main network switch

Couple EAP772 wifi access-points (powered via PoE ports on the switch)

All centrally managed via an Omada SDN Controller 6.0.0.25 (docker container)

[Issue]

A couple days ago we ran into an issue that Netflix refused to start streaming after selecting something to play. While it normally starts within a second or so, it now takes a couple of seconds before it shows a percentage indicator that slowly increases to 25% and then stops. When testing with my phone which is connected to the same subnet (but via wifi) I have no trouble whatsoever. 

A full factory reset on the Samsung Q8F smart TV did not solve the issue. After setting up it was immediately there again. Other internet based services like Youtube, HBO etcetera work fine, so I could safely say it's not the ethernet cable connecting the TV to the network.

After spending a couple hours diagnosing the issue and toggling many settings to find a solution, I finally decided to change the LAN settings and set the normally automatically configured DNS server to a manually chosen one and use 8.8.8.8 as DNS server IP (Google).

Directly after refreshing the network connection on the TV, Netflix started working as normal. No issues, no delays, instant playback! To confirm the issue I reversed the modified settings, resulting in a dead Netflix again.

Not a single settings was recently changed before the issues started. The only thing I noticed was an update to the Omada controller just before the issues started.

The issues seem to be DNS related and because I'm using local DNS entries, I can't just leave the DNS server set to 8.8.8.8.

Anyone who has experience the same or similar and has a clue what to look for or change?