AVOlio wroteDon't think HDR is related.
Netflix automatically detects the device on which you're watching or downloading, and if it supports HDR it will show the content in 4k HDR, likewise if it's 4k but doesn't support HDR it will show the content with a 4k resolution, otherwise it will show HD.
I have Galaxy note 4 (which is not on the list as well) it doesn't support HDR and max resolution it can handle is HD.
So I doubt it's related.
Well, if it is from the ISP side, try downloading a 10 min show on mobile network (preferably near the end of the billing cycle) or from another WiFi source at work, school or wherever and see if you still have a problem. If the issue is solved, it is something from your isp and if you still have an issue, it may be from the app itself---reinstall it see if that helps. Also, try it on another phone. Maybe the phone has some sort of sort of unknown incompatible issue which is preventing it from downloading at full speed.
Anyhow, good luck.
(and jealous of 24 mbps speed)