Apple Watch Series 4 pairing issue after dissociation

I dissociated my Apple Watch from the my phone in order to sell it. I paired a new Apple Watch Ultra ; all fine, without any problems. I tried to pair my Apple Watch Series 4 with another iPhone, but due to unknow causes all icons in a state where 90% not available (i.e., they are not complete in their full circle graphics) with only "settings" available and a few other features. I have already followed all the stand-alone maneuvers on the watch step by step, but I cannot connect it to anything. I took it to an R-store (Italy) and they told me that if I want to have full available again , the cost is €350, but the actual cost is less than half. Has anyone, kindly, ever found themselves in this situation where you can neither go back nor move forward? The reason given by the technician is a HW problem, but I'm not convinced.

[Re-Titled by Moderator]

Original Title: WATCH 4 SERIES - BLOCKED