Netgear lb2120 connection with external antenna

I’ve bought a external antenna to boost the signal coz I live in a wacky area, the 4g plug that came with it is too big to fit into the modem, is there a way I can buy smth like an adapt or to make it fit? If so can you please guide me which one would work, thank you. I’ve tried going with the modem to nearby hardware stores and see if they have smth for it with no luck. Uk