Hi there everyone!

I am writing back to thank all of you for your help and contributions, and to give you some news.

I didn't expect to spark such a rich conversation!

In the end I tried one or two more times, with the advice of some of the answers here. It still didn't work.

I then bought a large patch, also didn't work.

I ended up buying a new inner tube.

Some caveats though:

First, my conclusion in the end was that probably, the reason why my homemade patches were not sticking was that the surface of the inner tube had been scorched too much, or had remains from earlier tries or something, since even the bought patch didn't hold at all, and since conversely, my homemade patch on the other side of the tube, where the nail had pierced through, held fine at all times.

Second, I would still moderate a conclusion that buying a new inner tube is much simpler. In reality, I bought a new inner tube, started riding, and it literally blew up after 10 seconds. Probably because my tire is a very old one, and the shop I went to only had inner tubes that were for bigger tires. Still ok, but the seller told me to be very careful when installing it. So I had to buy a second inner tube in the end, and had to thus change it twice. So going for an inner tube in my case didn't end up being that simpler. Especially if one takes into account that to go the homemade patch route, I didn't have to leave my home, whereas to buy the two inner tubes, I had to ride twice to the shop, which took me half an hour each time.

So I still conclude that trying a homemade patch is worth it. In my case it didn't work, but it probably could have worked if I had tried correctly from the start, and I am pretty sure it would have worked if it was a normal hole, and not a tear.

Thanks a lot very much for all your advice, reactions and help!