Big troubles

Sounds a lot like a bad HV battery. You really need to get the codes read by someone with Toyota software or buy a mini VCI and read them yourself. The inverter coolant pump reservoir should also be checked for swirling coolant inside with plenty of movement. If you will post the DTCs on here, someone will help or point you in the right direction.
Where are you located?

 

First thing you can try is to take it to a local auto parts store and have the codes read. Most places like Auto-Zone will do this for free. This won't give you all of the codes (It only reads OBD codes, not Toyota ECU specific ones), but often it's enough to have a good idea what has gone wrong. Then google the codes for the Prius. I agree that it sounds like a hybrid battery failure, but the codes will confirm.

 

Since you took your car in to a mechanic to have the aux battery changed, I'm assuming that you don't do your own repairs. So we're trying to give you leads to suggest to A mechanic for them to check out. A question for you, is your mechanic conversant in all things Prius? Any old mechanic that's fine on standard vehicle computer systems, engines, etc. may not have the knowledge, skills, or tools to try to fix your Prius. The problem is that everything on a Prius interfaces with some kind of ECM/BCM/PCM/ or other computer. It takes special hardware and software to get in an read these magic boxes. Get the picture? A whole different kettle of fish.

And one can get seriously dead if one doesn't do all the safety stuff with the traction battery--which can put out almost 400 volts.

NAPA can get Dorman's remanufactured battery for about $1399 + tax, and there's a core charge. https://www.napaonline.com/napa/en/p/NHY2001001/NHY2001001_0199717741

The traction battery may be the problem, yes it might. There are fixes, of course. But a run of the mill mechanic is likely to just plop a new/remanufactured battery pack in (up to $2 grand in parts) and send you on your way. The battery pack may only need to have the buss bars/ connections between cells cleaned--they can get corroded. And then have the cells rebalanced so that they're all working together. My 03 got this procedure and is working beautifully.

My suggestion is to seek out an independent shop that knows Hybrids. They are more likely to think and fix, rather than just change parts. You can probably find one on Google.