"connecting my PS4 directly to the modem, but even then my ping is unacceptably high." points to ISP or ISP gear issues and not your gear. Download speed is one thing, ping time is another. You can't infer one from the other. Example? Satellite ping times are very very high but download speed can be over 1 megabit.
To me this points to an overloaded ISP. You can try another DNS and game server but your story points us back to the ISP or their modem.
As to no clue, the clues are here but I find folk want a fix for their ISP. Do I need to write more?
I do need more from you. Which ISP?
So where do I start this off. The last couple of weeks/months, I have been having some serious issues with my internet connection. All of a sudden, the connection has been way more unstable and slower, for no particular reason because no new devices or such have been added to the internet. So I don't really understand why it all of a sudden took a turn for the bad. The thing that's been bothering me the most is my connection on my PS4, because when gaming the latency and ping is the most noticeable. Before it went wrong, I would get anywhere between 25 and 55 ping while gaming on my PS4 and my connection was pretty consistent. I use a powerline adapter to connect my PS4 to the internet with cable. Now these past couple of months, I would say I have 70 ping on average, but it sometimes goes as high as 250ms and overall it is just really unstable. I have tried connecting my PS4 directly to the modem, but even then my ping is unacceptably high. My internet provider has come 2 times since the trouble occurred, but every time they would say the connection looked pretty fine and ofcourse when they came over the connection with my PS4 would be fine. Even when no other device is using the internet, my connection on the PS4 is really bad. The problem also doesn't really come from slow internet, because my download speed is around 45mbps, which should be enough. It's just really annoying and I'm pretty desperate right now because I have no clue how to solve these problems. Any help would be greatly appreciated!