Just returned from Rome yesterday and hoping to add some clarity to the Coopculture tickets. Ticket sales open one day at a time, 30 days ahead, and they can go fast.
You can visit the Underground ONLY with a guide. I visited a few years ago, and so much has been added since June. There are extensive walkways throughout the underground and reconstructions of the lifts and trapdoors for animals. Also a new museum I don't remember on the upper level. Highest level (3?) in not open at the moment, but was memorable most for the view over Rome as I remember.
We bought the Sotterranei Ingresso Orario tickets for 24 euro. I thought there was another ticket for 32 euro, and I was a little concerned until we got inside that I might have bought the wrong one. Our ticket got us an English tour with only 6 people and an enthusiastic guide. I think the max is 20 or 24 people. She took us specifically to the underground areas and we were on our own for the upper levels, which was perfect for us since we've done several tours here with different guides over the years. She suggested we download the audio tour for the upper levels, but that was not possible because there was no WIFI. I think a downloaded RS tour would be fine. The ticket booth where we checked in also rented audioguides, but there wasn't time to go back there to get one once we realized we couldn't download anything. So do it ahead of your tour if you want the official audioguide.
If you want a guide for all the levels, keep looking on the website until you find the one for 32 euro.
We chose the first tour of the day when the gates opened at 9:30. Our guide spent so much time with us she was late for her next group! This is a great experience. And you will need your mask and CDC card to get in.