What is GPS RTK?
How Do I Get the RTCM Messages to the GPS Receiver?
It depends on your end application. If you need maximum portability, then the best solution is a radio link between base station you've created and mobile GPS RTK receiver. SparkFun offers a variety of LoRa radios and antennas to enable this backhaul like some of the products listed below.
SparkFun MicroMod LoRa Function Board
WRL-18573LoRa 1W Breakout - 915M30S
SPX-18572If your end application already requires an Internet connection such as GSM or LTE-CAT, then a serial connection to the GPS-RTK receiver may be the easiest way to go.
If your application has a cell phone nearby then a third option is to create a serial bridge from a cell phone to a serial Bluetooth device like the Bluetooth Mate Silver that then connects to the serial port on the GPS-RTK. There are a few NTRIP compatible mobile apps. We’ve been pleased with Lefebure for Android.