This page allows you to calculate the exact frequency from the channel numbers for 5G NR, 4G LTE, UMTS, GSM, CDMA and iDEN networks.
In order to obtain the ARFCN/UARFCN/EARFCN, you will need to enter "Field Test Mode" in your phone. This varies greatly from phone to phone. In general, all involve dialing certain special codes in the dialer in order to bring up the hidden options.
(phone to base station)
(base station to phone)