It is strange for the throughput to be so low on the PS4 Pro if it is connected via Ethernet?
How are the speeds on other devices? If you are getting significantly under the throughput you are paying for, then chances are that you may be on an oversold node from the ISP in which case, you will get congestion related ping issues that can't be fixed at the router, level, especially if the ping measurements are done when your specific WAN connection is saturated.
If throughput remains low, I recommend restarting the cable model, and then checking the status page to see each individual channel to see if everything is properly stable and that the SNR is not too low.
If the SNR is too low, inspect your coax splitters, and make sure you are not using an unnecessarily large one, e.g., I have seen cases where someone would have a 6 way splitter with onle 2 or 3 ports used, and the remaining ports not terminated, which lead to a high error rate and poor SNR. Finally if you are on an oversold node, call the ISP to complain periodically in order to see if they can improve the situation.
