Slow internet due to distance?

With me there's the following problem.

I've had very slow internet for some months, the router is located exactly one floor below me, there I measured about 70k download and 7-12ms during a speed test (a year ago, half a year ago and several days ago). In short, the speed stays the same. On my father's laptop we measured 60-65k, but this difference is not a problem. However, a year ago we had only 15k-30k download and 20-25ms measured on my PC (NO laptop). At first it was OK, but I still did not like this big difference from the download.

In any case, it was still manageable. Since about May of this year, the internet has become quite slow. I measure on good days (when I'm the only one with a device at home just 7k download and 60ms, the 'ms' often increases to 300 or even 1000). But if also a family member with a device bsp. Mobile comes online, the download drops to 2-3k and sometimes to below 1k and the ping does not fall further than 300ms!

The router is quite new (1 year straight) why my dad does not really feel like getting a new one, which is understandable.

The router is from the telecom, below I attach a picture of it, because I have no idea which model that is. Moving the router to my floor is almost impossible due to the many cables.

I also use a Wi-Fi stick, but I guess that should not be because I've been using the same for a year.

In short, my question would be

What could I do about it, except buy a new router.

P.S: W-Lan repeaters do NOT do anything, on the contrary, they slow it down even more. (tested over 4 pieces, from cheap goods to expensive ones.)

This is because the floor / wall attenuates the signal because there are no repeaters

there you have to lay a lan cable

Is that the only option? With us it is not very easy, you would have to drill a hole in the ceiling with us, to lay over the stairs the LAN cable would be even worse.

Try it in the router to change the channel for the Wi-Fi! If the house is not alone "on the other side of the hall", maybe a neighbor has installed a new WLAN that bothers yours. This is noticeable only with weak signals - so only with you above.
You should also take a look at the cell phone, at which point in your room, the Wi-Fi reception is strongest. With a USB extension cable, you can set up the Wi-Fi stick there.

So first of all, learn to distinguish between the Internet and your local network, because your Internet access has not slowed down.

Wi-Fi is radio and radio is unstable.
Alone the ISM band used is varied and not only used by Wi-Fi.
The transmission line of WLAN access points go from 25 milliwatts to 1 watts and can therefore be easily disturbed.

If you need to use Wi-Fi, then you can improve it with a Wi-Fi network that MESH and MU-MIMO dominated. But since you apparently have no hardware so far, the question arises whether you for the 200 to 300 euro, which would be appreciated, not just a LAN cable can be installed.
This would give you the fastest and most stable connection.