If you are looking to try out 5G Standalone with SIM ONLY plan. The only non contract option available is from M1.
Thanks to the latest MIUI update from Xiaomi, the latest firmware now adds the 5G SA option which was not available in the earlier firmware.
With the new 5G SA option enabled, we are now able to connect to 5G SA network and make calls over VoNR.
Speedtest of M1 5G SA network