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.
If 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.