@Miguel Herranz
Yes, I have used red light therapy to very positive effect. I have a chronic ankle injury with grade 4 arthritis (bone on bone), and red light therapy has made a huge difference in pain and function. I can't personally vouch for any of the other purported benefits, but I have honestly been shocked at how much of a difference it has made for my ankle.
I was sceptical about it at first, so I started with a small inexpensive unit, that is basically a big light bulb with 12 LEDs. It worked so well that I got larger panels. The bulbs work okay, but since they are low-powered, only at relatively close distances where they don't cover much area, and they are more awkward to use.
The website you linked to is a good source of information on devices, usage and dosing.
As far as devices go, I'm not sure about availability in Spain, but the article makes some recommendations. Although Joov has been the leader in promoting and marketing red light therapy in the US, I would stay away from their devices. They are overpriced and I find some of their marketing disingenuous and misleading.
Redtherapy.co and Platinum LED offer much better value.
However, I have found a much cheaper source for red light therapy devices.
There are a bunch of Chinese companies that make LED panels and sell them on Alibaba.com at wholesale prices (even with expensive shipping, they are much, much cheaper than retail units sold in the US). One I have used and can recommend is called SunGrow. I tried them because the specs for their lights seemed comparable or better than the panels sold in the US, and there are a few vendors here selling panels online that are obviously sourced from SunGrow. I also found a Reddit thread where a guy had ordered from them and was very positive about it. SunGrow has their Alibaba listings set up so you can just order and pay for products through a shopping cart, and you can order single units. The email communication from the company, in English, was prompt and responsive, the shipping was fast, and I am happy with the quality of the panels.
The standard wavelengths for therapy lights seem to be 660nm (visible red light), and 850nm (near infrared, barely visible to the human eye). Supposedly, the visible red light penetrates shallower and the near infrared penetrates deeper. My panels are a mix of the two wavelength in a 1:1 ratio, which is pretty standard, although some companies offer different wavelengths and combinations. SunGrow will customize panels, but you have to negotiate that directly, and manufacturing time may be longer. IMO, the standard combination is fine.
As far as dosing, ideally you would get an irradiance meter and check the actual irradiance at different distances, then adjust exposure time and distance to get the dose you want. But the reputable panel manufacturers give irradiance specs at specific distances so you can approximate without measuring yourself. The site you linked to gives good dosing guidelines. There does seem to be a large range of effective dosing, so it doesn't have to be exact, but way too little won't have much effect, and way too much will also have no effect.
Staying close to the panel gives you higher irradiance and less needed exposure time, but you won't be able to cover as large an area (depending on the size of your panel). A larger distance from the panel gives you lower irradiance, so you need a longer exposure to get the same total dose, but will cover a larger area. Some studies, especially for skin conditions, indicate that there MAY be a greater benefit to a longer exposure at a lower level of irradiance, for a given total dose. I tend to keep the panel as close as possible, just for the convenience of needing less exposure time.