How can I go about becoming a range cadre as an AGR position?

In the Wisconsin ARNG, you can find AGR positions and apply for them here:
https://dma.wi.gov/DMA/humanresources/fedhr/agr
"Range cadre" is not an AGR thing.
Units run their own ranges. Training Installations have full-timers that run Operations, they help units schedule ranges and training areas, and provide training for Range OICs.
Those full-timers are rarely AGR, as we do not get a managed level of resources to allocated AGR positions to that for installations. It is usually ADOS, tech or state active duty.

Check with your full time AGR, personal, they should be able to help u out.

I don't even know if this is something that AGR fills. Those are usually Active duty or civilians.
The AGR page is on the HRC website and has all the information for submitting a packet for AGR consideration.