You probably already tried one.

A small white plastic box, Amazon Prime delivery, a few hundred reviews that looked decent enough. You plugged it in. Maybe it seemed to do something the first week.

Then nothing. The fleas came back and the device just sat there in the outlet, blinking its little blue light, doing absolutely nothing.

So you concluded that ultrasonic repellers don't work.

That conclusion is almost right. Those specific devices don't work. Not because the technology is flawed, because they are calibrated for the wrong animal entirely.

Every generic ultrasonic device on Amazon emits between 9 and 15kHz. That is the hearing range of mice and rats. 

The manufacturers are not lying, those devices do work on rodents.

But a flea hears between 25 and 66kHz. To a flea, a device emitting at 12kHz is completely silent. You might as well have nothing plugged in.

There is a second problem. 

Even a device operating in the right frequency range will stop working if it emits a single fixed frequency. Fleas adapt. In two to three weeks they learn to ignore a stable signal. 

That is why people report that even better devices seem to work at first and then quietly stop.

So what actually works?

A device calibrated to the exact frequency range fleas perceive — 25 to 66kHz — that shifts frequency continuously. 

No stable signal. No adaptation possible. The environment stays hostile not just for the first two weeks, but permanently.

That is not a different version of what you tried before. It is a different category entirely.