Fix ps4 bad internet connection?

I have had a good internet connection every time since I got a LAN cable and it has become very unstable for 2 weeks. I connected the Ps4 to another television 2 weeks ago to see how it can be played on it, however Without Lan cable but with Wi-Fi. Then I closed the Ps4 and the Lan cable and the original place with the good internet connection and noticed that you connection was very bad. That changes from day to day, sometimes bad sometimes good.

So first of all, the TV does not matter for the internet connection of the PS4. It is important whether WLAN or LAN. If you can connect the PS4 to LAN, you usually have the best possible connection. If you still have "fluctuations", then it is probably more due to the router or the network or to too many users.

Check whether your PS4 connects via LAN or via Wi-Fi.
You probably only have to reset this.

Check all connections and also the LAN cable. Try another one. Otherwise, it can also be the provider, log in there and ask them to test the lines.