So, that depends on your definition of “civilization” because there are little trading outposts and even villages that don’t have roads but you can get to by off road/4x4 vehicles and snow mobile, those are obviously closer than cities with “modern services”. I’ll try to summarize and hope it answers your question!

-In Russia, essentially, Irkutsk (pop 600,000ish?) is the closest “city/modern services” (even though Irkutsk is not in Siberia I had to go through there due to weird visa/permit reasons, though there are a few cities of this size all about the same distance away). As the crow flies that’s about 1,100mi/1,770km away from where the nomadic tribes are. After Irkutsk it’s only small villages without paved access or even smaller outposts. I went at the time of the migration to the winter grounds, which puts the herders going within roughly 100mi/160km of a trading outpost. That’s pretty much the closest they go. I got a snow mobile ride just over half that distance, and then had to ski the rest. That ski took 3 days, and on the 4th day I found them. On day 3 I knew I was close because I found sledge prints with reindeer prints in the snow.

-In Mongolia, from Ulaanbaatar I went to Olgii (pop 20-30,000?) which is arguably the closest “city/modern serve center” to the nomads I stayed with. As soon as we left the city center of UB we were off-roading. On maps there are roads, but they are not paved and not even grated (as of 2015). I got a smaller Jeep in Olgii toward the hillsides where I waited. The jeep ride was probably 80mi/130km as the crow flies, to where I was picked up via horse. I was picked up by the dad and his son-in-law, they had diverted probably 10mi/16km from where they were on their big seasonal migration route. That was easier than Siberia! I only waited for about 4hrs.

There were no real issues either time. Siberia was more worrisome for me. I didn’t know exactly my “goal” other than a very broad area (basically I was aiming for a lake they often travel by +/- 100km) and we were guessing based on the weather the prior month. So I was incredibly lucky, and had very good guidance from a local hunter/trader. By the time I got to Mongolia, I was like, ‘just wait on a hill and they find me?! Awesome!’