The short story, you need a tech to inspect and possibly replace your external cable and its connectors.
The longer story .......
Very simply, you have a signal level problem. The chart shows the DOSCIS 3.0 signal levels, channels 1 to 24 from your data. That line should be a flat line at 0 dBmV, not dropping down to -10 dBmV. In your case, that data almost doesn't matter as the modem is running DOCSIS 3.1 on the downstream side and is most likely not using the DOCSIS 3.0 channels. That depends on whether or not the modem is running DOCSIS 3.1 on the downstream side, neglecting any DOCSIS 3.0 data, or possibly running in a hybrid mode, using both DOCSIS 3.0 and 3.1 data.
In your case, Eastlink has configured the DOCSIS 3.1 OFDM channel to start at 695.6 Mhz which is at the far right hand corner of that curve. The occupied bandwidth runs from 751 to approx 844.95 Mhz, even further to the right. So, at that point the signal levels are dropping off of a cliff. That would explain the poor downstream performance. Its possible that the modem is having a hard time processing the OFDM data, and might be switching back and forth between the DOCSIS 3.0 and 3.1 data. It all depends on the limits that Eastlink is using.
Food for thought, its typical on many ISP systems to see a high frequency roll off starting at 750 Mhz, which means that the signal levels drop even further at or above 750 Mhz. That's due to older equipment in place that isn't designed to support frequencies above 750 Mhz. That's a reflection of the design of the cable system to support analogue cable tv. To support higher frequencies, the output cards of the neighbourhood node, any amplifiers between the node and your modem, and the local tap have to be rated for higher frequencies. The local tap is located at the nearest utility pole, or in the case of underground cabling, inside the nearest pedestal, which should be visible from your front door. That tap looks like this:
High grade.
www.acmetronix.com
You and your immediate neighbours will be connected to a local tap.
Given the higher frequency range of the DOCSIS 3.1 OFDM channel, Eastlink should have ensured that all of the cable system components in your local area support frequencies above 750 Mhz. If not, that high frequency roll off will be a problem for the OFDM channel.
Call Eastlink tech support and ask them to run a signal check on your modem. I'm assuming that they have an automated display that should simply indicate a pass/fail result. The result should indicate a failure. For the Hitron modem, there is a signal display available that breaks down the OFDM channel into 10 or 12 sub-bands. The might not be available to the front line techs, but you can always ask. If not, the only data that the tech sees will be a single line of data, just like what you see in the OFDM data line. The OFDM channel signal level might fail on two accounts, simply for low signal levels, and for an excessive downward slope if the signal level drops even further above 700 Mhz. There isn't enough data available to determine that, but, if the tech has an OFDM sub-band data display, that drop would show up on his or her data display. No matter what, given the signal data and the signal drop-outs that occur, the tech should agree that there's a problem that requires a field tech to check out.
The upstream frequencies run under 50 Mhz, which is on the left hand side of that chart. Those channels are a little high, normally their in a 36 to 40 dBmV range. If the tech solves the issue with the cabling and its connectors, I'd expect to see those signal levels drop into the 36 to 40 dBmV range. Where they are now isn't a problem, their just higher than I would normally expect, but, given the downstream signal issues, where they currently sit does make sense.
