How in heart is possible to work with a router that randomly shutdowns itself around 3/4 times a day without any reason. Leaving me without internet for 10-15 minutes. I called my ISP and they said that they don't see any problem and they will not send any replacement. I can instead fix an appointment with their technician to have a look at the issue, but of couse, I'm paying for him.

How about no? I'm going to buy a proper modem/router next month, this shit router they give you have 1/100 of the proper configurations options you need.