Using WiFi Antenna for 5G Mobile Network?
Antennas work on resonant frequencies, which require an antenna design that targets the specific frequency range and impedance for the transceiver.  Wide band antennas are rare over a large range of frequencies, and those usually have very low gain, since they can't really be tuned to a specific resonant frequency.
Most multiband antennas actually contain a series of more narrow band antennas, that are either combined on one feed(much like an old-school UHF/VHF television antenna), or have separate coaxial feeds for each internal antenna, which need to connect to separate ports on the transceiver which will feed each antenna signal to either multiple individual transceivers, or a diversity switch and filter combination that feeds a single transceiver.
Every multiband antenna I have seen recently is only resonant within the specific range of the three or four bands that it tries to cover.  Outside or between those frequencies, the antenna will have almost no gain in comparison to the gain of the elements that are in-band.
TL/DR - A generic 2.4GHz-5GHz antenna will perform quite poorly trying to transmit or receive on the bands between or outside of those two frequencies...