To configure the external radio, you will need to ModemVU. Currently NetView and Modem cannot configure a standalone modem (I've already requested that this feature be added to NV&M). If you are using a USB connection between the radio and PC, use NetView and Modem to see what COM port to use with ModemVU as it will automatically detect the port while ModemVU will not.
In ModemVU, you will need to change to these settings (be sure to click apply after making any changes):
Of course, you can use whatever frequency(ies) you like. You want to make sure that the Rx frequency matches the Tx frequency of the base. You want to make sure the rover Rx frequency matches the Tx frequency of the repeater. It is possible for you to use the same frequency for base, repeater Rx, repeater Tx and rover and the rover will detect both incoming corrections and select the strongest for the RTK engine(s). However, you may wish to select one frequency for base Tx and one frequency for repeater Tx as this will allow you to know, based on the frequency selection at the rover, whether you are receiving the corrections directly from the base or from the repeater.