Do you have a device that you can connect directly to the modem with a LAN cable? If so, please see if doing those things on that device gives better performance.
While trying this, please disable the WiFi in the HT2000W modem. If you do not know how to do so, please see Section 6 in this PDF. Please be sure to click "Save Settings" after unchecking SSID Enable for each radio rather than unchecking SSID Enable for all four radios and then clicking "Save Settings" (i.e. disable each radio individually, but make sure to disable all four).
Trying Youtube or Netflix this way will ensure that asolutely nothing else is sapping your bandwidth at the same time. You mentioned that it's eating data like crazy, and this could be a sign that something else is sapping bandwidth at the same time that you are trying to watch Youtube or Netflix.
Also, if the definition you're trying to watch Youtube or Netflix in is 480p (SD) or below, the following shouldn't matter, but you could also try pausing or turning off the Video Data Saver to see if it makes any difference with the buffering.