I have operated a rover on an athletics track using an on-board ESP32 running an ntrip client connected to a wifi hotspot on the phone in my pocket: RTK vs M8N Comparison - Rover
If you are on a budget and want a GNSS with built in multi frequency RTK then I have had good results with: http://navspark.mybigcommerce.com/ns-hp-gn2-px1122r-multi-band-quad-gnss-rtk-breakout-board/ for $125USD (+ antenna)