I solved the problem and it turned out to be a hardware issue.
Presumably, there was a "detachment" of the Intel 6E AX211 chip, possibly due to overheating...
On the Dell Precision 5570, the 1216 form factor chip is soldered onto the motherboard. I was only able to buy an AX201 as a replacement. However, I first preheated the old chip on the motherboard with flux. Of course, the protective casing made of thin sheet metal was removed during the soldering process. After that, I reassembled the laptop (with the old AX211 chip) and, lo and behold, everything worked as it should. According to the datasheet, these Wi-Fi chips should withstand up to 80*C.