walkie talkie yup. Or even portable two-way radio station.
I remembered a movie where a student from the 40's builds a campus radio with almost nothing... and google tells me it's a foxbole radio.
And Senku already has all the bits and pieces available.
Two ways isn't even that important.
And I can see foxhole radio working quite well outside the buzzing world we live in.
Even a morse radio would suffice and you can learn morse quickly.

In fact it's far more feasible than the antibiotics route he chose...