Key fob stop working. no battery issue.

I just bought this new for me 2009 Prius with 160k miles.
When I got it, the key fob was working fine, proximite to the car, the door open, or I could close it using the keyfob to close/open the doors as well.
I drove it from the previous owner house using the keyfob on my pocket, so no need to put on the slot.

I drove to a car mechanic, to do few thinks, like clean up the car, replace 4 tires, and do a full checkup on the car, oil change, etc. etc.
The car was there for a full week.
When I went to pickup the car, the key fob wasn't working. New to Prius as I'm , I thought: "The key fob battery die"
I drove it, opening the car with the key, and driving it with the key fob on the slot. No problem.
At home I immediately open the key fob and replaced the key, even do it had the red light and clicked when pressed any button, but replace the battery any way.

No luck!!! still not working to close/open the doors neither via proximite nor pressing buttons.
I came here and read a lot about similar problems and follow some tips, but no luck still.

I checked the button underneath the steering wheel. Was OK.

No stuck buttons on the keyfob

Not sure how I could test the 12V battery on the rear of the car, but the car starts and all lights are OK even with the engine off, so I'm assuming that battery is OK.

I'm trying to use it far from my cell phone

I have only one fob, so I can't do what is called "Chicken Dance" with a working one.

So, I'm assuming I'm running out of options, but anyone know where else I could look to fix it?

Thank you
ehm

 

Would be very hard to prove that the mechanic did something.
I didn't drop it, but who knows what happens at the mech.??
So, basically my only option will be to to the dealer? Only them have the equip. to check it?

 

Returning here to update:
I give up and went to the dealer.
$160.00+$60 US dollars and almost 3 hours later, I have a working fob.
My original one, according to the dealer, was death. I bought an used one, and they program that one.
Not happy with all this. for a such low priced car, dealer charged just a bit less then a Key Fob programming from BMW dealers. And 3 hours??? the guy was manually coding the keyfob???
Not happy, but it's working now. The old one still work as a mechanical key. (inserted on the slot, works)
Thank you All