Telekom Slovenije is the biggest operator in the country. They are market leader with a 45% share in 2016. Their 4G/LTE is accessible for prepaid and covers 98% of the population already: Coverage map. It has frequencies on 800, 900, 1800 and 2600 MHz (B20, B8, B3, B7). It’s the first provider to offer 5G, but not yet for prepaid.

Telekom Slovenije Paket MultiSIM MOBI

manuals: https://cutt.ly/mobieng

manage SIM online: https://moj.telekom.si/

Top-up is possible using

moj telekom: https://moj.telekom.si/

and

 pay365 (use google translate) : https://www.pay365.si/spletnapolnitev