What connector is this? (Inside a wifi modem)
It's a coax receptacle with an internal N/C switch.
The N/C switch brings a 50Ohm connection between the onboard/default (e.g. PCB trace type) antenna and the RF transceiver (Bluetooth, Wi-Fi, 2G/3G/4G or whatever). When an external antenna is plugged into the receptacle, the default antenna gets disconnected from the transceiver. It also enables a connection to measurement equipment (e.g. spectrum analyzer) to measure the transmit/receive power.
Murata's SWF series and Hirose's MS-156C series are quite popular.