Most of the metro is covered, and if you have a Russian phone number you can register to use it for free.
Parks have surprisingly high wifi coverage, especially in Gorky Park and Sokolniki Park. Here is a full list of parks and their wifi coverage.
That’s it for the free options, but if you need 3G all the time, you can buy a local pay as you go SIM card and put it into a smartphone, then use it as a wifi hotspot. It’s very inexpensive, the speed is good, and all of the carriers have these SIM cards.
For those who need 24/7 coverage on the go around town, YOTA 4G mobile modems are the best bet; but the priciest. It’s only worth it if you live here or are staying for a while.