You can make your own antenna and cut a single wire to length with an easy calculation

Wavelength = speed of light / frequency
(λ = c/f)

Do a quarter-wave length vertical wire, like 22 gauge or something. At 915MHz should be around: 3.2248248772420216 inches.

Not sure whether you can flash the firmware or if the modem is an sdr or different unit for each spectrum.