Jitsi works, and they have open relays to test with, but as the thing here is very much analog and I'd assume she'd just need to see your position, how hands move etc, the audio quality isn't the most important thing here. Sure, it helps, but personally I'd just use zoom/teams/hangouts/something readily available and invest in a decent microphone (and audio in general) + camera.
That way you don't need to provide helpdesk on how to use your thing and waste time from actual lessons nor need to debug server issues while you've been scheduled to train with your teacher.
Guy reporting to Putin and Ukrainian chiefs have quite opposite interests to give estimations and I don't know how accurately you can draw lines on the maps, so I'd guess that the russian guy underestimates the area by a lot in a hope that his window doesn't suddenly break with his body and Ukraine likes to give a round number, even if reality is 830km² or whatever (I don't believe that they'd publicly lie about the area to be 3 times bigger than actual, but 20% on those cirumstances is pretty much just a rounding error).