It isn't an under the table thing, at least in the UK. For the generalloy used frequency bands for this, there was an auction for the spectrum. You can get access to parts not being used by the people who bid on it.

You can also get an amateur radio license, and use frequency not normally used for this, but that probably means using SDR as most devices designed to use LTE will only work on normal LTE bands etc.

No bribes, connections, or anything shady etc needed.