I can't advise on the M8N compatibility, sorry..
RTK needs a ground station - somewhere...
If it's close to you, you get better accuracy, but using a public one miles away is better than nothing.
I've set my own up, as I was already using a Raspberry Pi & a ublox gnss board with outdoor fixed antenna as an NTP reference, so adding the RTK part was quite easy.
I used this:
with the Ardusimple ZED-F9P development board
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(Though mine cost a lot less, from another reseller!)