How many intact and tactically relevant cross border transmission lines between Ukraine and Russia can there be?

Use the transmission lines to link up with an RF node to hand off. Friendly territory might just be some place without good Russian internal security coverage (perhaps deeper in Russia rather than towards the front lines if the node uses satcom).

A friend of mine worked on a covert comms system for the Rangers to use in the Battle For Berlin that thankfully never occurred. The idea was to clamp onto plumbing, fences, and similar infrastructure where possible. Nodes with radios handed off to other comms systems. It worked reasonably well in tests but I don't know that it went anywhere, point is that the theory is sound.