There isn't really one. This is a system that's supposed to have it's own built in redundancy and must not go down.

Flight Service uses a different set of radios and can relay messages to ATC, but they've been slowly dismantled over the last two decades and wouldn't have the capacity to step in for an outage (nor should they be expected to - that's not their purpose).

Some aircraft have CPDLC for text-based communication, but it's asynchronous and you can't use that as a replacement for voice communication in the terminal environment. Also, many aircraft aren't equipped to receive it.

Pilots are trained on lost comms procedures, so if you're in the air and the radio fails they'd fall back to those regulations. Basically all the aircraft would fly their last assigned clearance, and attempt to deconflict manually with other aircraft over air-to-air radio (similar to flying into an untowered airport).

I would bet the problem is not strictly the radios themselves, they will still work, Planes can talk to each other and directly to airports, But that there is a large scale highly integrated control center network that failed, and planes can no longer talk to centralized traffic control.

So it falls back to local control, more in the style of visual flight rules. Pilots in existing flights will have to be vigilant and do their own deconflictation and new flights are banned out until the network is back up.

All conjecture at this stage of course, but all the radios won't just fail. It is probably going to be a poster child example about the dangers of large scale integration. The attraction of course is that integration is very efficient, the danger is it tends to fail all at once instead of in smaller pieces.

In the worst case: light guns and wing-rocking or flashing the navigation lights.

Technically, you do not need radios. There are many many uncontrolled airports in the United States, where you can technically fly around without establishing two-way radio communication. It’s doable, but I don’t know if that’s even a scenario pilots test in the simulator during their semiannual trainings.

AM Radios are literally everywhere, and very commonly available.

I would hope that airports keep a battery operated radio + antenna in case of emergency with them, to at least communicate with aircraft within visual range of the airport.

Multiple independent radios, for starters.

There is CPDLC or "datacom" that can be done via satellite, but I think most planes that have it, use it via ground-based radio.