Why aren't tethered balloons used instead of Cellphone towers?
One of the reasons, I'd say:

A cell phone tower holds many antennas. Need to do maintenance, you go up, fix a given antenna.

Your balloon has many antennas. One needs maintenance. Do you bring the whole balloon down? Find a flying technician?

(And if a balloon cannot hold many antennas, then maybe just build a tower instead?)