I have a few ideas.
Any radio antenna outside the dome would eventually fall into disrepair with nobody going out to fix them and fail. Antennas inside the domes would be attenuated by the dome, which would presumably be built with metal parts to create a Faraday cage as well as just plenty of mass to diminish RF power. Increasing the power to break through could threaten people's health in the dome (as in turning the dome into a virtual microwave oven), interfere with vital equipment (medical equipment, internal radio networks, power distribution, etc.), and perhaps other issues.
There is such a thing as ELF radio communications through the dirt and water. This requires very large antennas, a lot of power, and even then achieves very low data rates. I would think that given the overwhelming desire people have to communicate, and that this problem is not likely to sneak up on people, that plans would be made to maintain these systems even with considerable cost. Cutting people off will be difficult as ELF communications is well documented and anyone able to maintain a city sized dome should be able to build antennas and transceivers. It would take a lot in my mind to get people to abandon this as a means to communicate, even if it was at a data rate measured in bits per hour.
After some thought on this and reading some of the other answers I believe it's just too easy to build an effective ground penetrating radio communications network. Ham radio operators are playing with this kind of equipment now. https://en.wikipedia.org/wiki/2200-meter_band
I'm thinking that people would have to be afraid to transmit anything. I believe a better answer is a combination of what others came up with. Consider a non-biological virus that is attracted to RF energy. Transmitting anything attracts the virus to the source. The early attempts to fight the virus left behind long lasting RF generators in locations far from population centers that pump out RF noise like crazy. These would be powered by solar power, wind, hydro, nuclear batteries, or anything else that they could contrive that could keep going unattended for decades or even centuries. This keeps most, but not all, of the virus around these locations but makes radio communications difficult worldwide.
Even that might not stop people from trying. And succeeding.
There's no people outside the domes to pose a threat, right? So this isn't a case of a zombie apocalypse of people roaming about looking for domes that could provide clothing, shelter, and brains to eat.
We have radio modulation systems that can operate below the noise floor. This makes the radio source difficult to track for intelligent humans, a mindless virus that is attracted to RF energy could be easily fooled with RF noisemakers while radio network can still communicate. Such communication schemes and frequencies are standardized and known by militaries, ham radio operators, airlines, and so on. Someone is gong to know how they work, and for people that don't these things are written down in books.
A virus is not likely to stop people from communicating by radio. A non-biological virus attracted by RF energy isn't likely to either. Can this virus turn people into zombies? Radio wave seeking zombies? Can the RF seeking non-biological virus eat through the dome materials? That might scare people enough to stop trying. Maybe.