If you're having packet loss and your max speeds are around 300mbps, that sounds like a line quality issue. Maybe check the modem connectivity section for correctable and uncorrectable. Depending on how long it has been since you've power cycled the modem, those numbers should mostly stay at zero. If you see spikes of those numbers at once then that indicates a link problem and probably needs a tech.
Also, I have bad luck doing tests against speedtest.net, I usually use dslreports.com as my goto.
PS: What modem do you have and how many channels down/up does it support (if using your own).
Final note: you pc's hardware can also cause some limitations to your connection. If you're not running at least a Ryzan or intel (i3/5/i7) series chip, your speeds may be limited by your cpu's ability to process the data, there is offloaded from the network card to your pc to improve speeds and performance, but if you have an old processor that can bottleneck things.