Ethernet vs RJ11 Cable for Orange Fun Box Router: Improving 80MB Internet Speed & 15m Distance

Hello, I have a fun box router from Orange. I have 80MB internet for the contract and the actual speed is 24-26. Already at the very installation, the technician noted that due to the fact that my home is quite far from the source 80 can not be counted, while he emphasized that replacing the cable between the telephone jack and the router can improve the speed of connection.

At the moment there is an old flat telephone cable from the socket to the router, I think he is calmly about 20 years old. My problem is that first of all the socket and router are in two different rooms at a distance of about 15 meters.
The second and most important problem is that I don't know at all whether replacing the cable with a better one (how is it?) Will even help? Is it better to move the router with the new telephone cable closer to the socket and invest in a good quality Ethernet cable and drag it to the computer (if so, which cable to invest)?

Thank you in advance for your help if I haven't provided any important parameters in advance I'm sorry I am a layman in this field

And here is the problem with what I do not know what cables I should buy to be sure that the lack of results is not caused by poor quality cables

Read the contract carefully, it probably says "up to 80MB, not less than 25" (I think). I had the same contract and these 25 were always there, although I am close to the source. Now I have a 100MB optical fiber and 50MB guaranteed. For you, the low transfer is probably due to the contract, not the cables. I used to have the "neostrada 40MB" contract and I always had 35 to 42MB.

The contract is always up to. And guaranteed I would not be surprised if it was lower. Check like a friend @szpoak he advised on the short RJ11 cable but I would not expect a gap between the results on the old and the new cable, 15m is not such a huge distance, you would really have to have patina tips covered to suppress the transmission.

Indeed, right with the contract. I will try even on a shorter cable because the one that is currently pulled from the router's socket is terribly old maybe it will be a slight improvement.