Congestion, which is heaviest in the evenings and on the weekends, can greatly affect one's ability to stream, especially in the higher resolutions, so your installer may well be correct.
With that said, if you haven't already, you may need to turn off, or pause, the Video Data Saver when attempting to stream in the higher resolutions. Keep in mind, however, that streaming in higher resolution uses a lot of data, and with you still being in the initial 20 days of continual data replenishment you're not going to know the real effect of HD streaming on your data allotment until that 20 day period is over. With that said, going by Netflix's amounts, per hour streaming in HD uses around 3GB, SD around 700MB and LD around 350MB.
Regarding what was stated above, for your first twenty data of service HughesNet continually refills your monthly data allotment. They do this as a courtesy, in order to allow new customers to update or upgrade their devices to current without it affecting their normal monthly data allotment, as those activities can use a lot of data. After those initial twenty days the continual replenishment stops and your data usage will be deducted from your monthly allotment in the normal manner. The twenty days of data replenishment is mentioned in the Welcome email sent by HughesNet.
If you find that you're always having trouble streaming, no matter the time of day, the reps may be able to help, but they'll first need some speed test results in order to determine if there is a speed issue. The protocol for that speed testing is here.
Thanks for posting and welcome to the community! I hope the information Gabe offered to address your concern helped. Haven't heard back from you so we're closing this thread. If you still have concerns, please start a new thread and include a detailed explanation so we can more effectively help you.