Radio technology is still available to the post-Rash culture, as the pretty much only modern/electronic kind of Information Technology, but its use is a lot more limited due to a number of reasons. Nonetheless, the Cat-Tank has a builtin radio station to establish communication back to the Known World - in partuclar, the HQ in Mora.
While the post-Rash humans have taken production and (battery) storage of electric energy to levels that are beyond the reach of our current technology, only the simplest devices - like the street lights in Mora - are affordable enough for common use. Electric vehicles are effectively restricted to military use, and devices that we'd call "electronic" - including radios, but also the control screens seen in the Mora HQ or at the Öresundsbro base - are said to be prohibitively expensive. This is likely due to the fact that actual electronic parts, from transistors to ICs, were mass produced pre-Rash in highly specialized plants, few of which were located in the Nordic countries.
While the massive enclosure and switch levers of the radio installed in the Cat-Tank agree with a design that fell back to the use of vacuum tubes, we have seen a walkie-talkie on a guard in Björköfjärden whose portability suggests that it was built with scavenged pre-Rash electronics.
In terms of modulation and other technical details, the limited availability of parts to build radios from should provide a lot of pressure to fall back to simpler, older variants like AM or FM. It is, however, noteworthy that no morse key has appeared so far.
Use Cases Edit
Short-Distance Communication Edit
As expensive as that walkie-takie must be, it is unclear why the Björköfjärden guard would need such equipment in the first place; Björköfjärden is a small, crowded, fortified place where shouting out an alarm is likely to be heard anyway, and connecting places of interest with speaking tubes seems possible. We would have to conclude that these devices continue to offer a particularly low-maintenance, hassle-free means of communication.
Emil mentions on that "a public radio system" is in existence. Considering the absence of working radio communication over longer distances, it seems likely that he's referring to a short-distance system, probably covering only Mora.
Medium-Distance Communication Edit
The abovementioned use of radio equipment stands in sharp contrast to the properties that radios (possibly intercoms as well) have exhibited on the Dalahästen and the Cat-Tank. It seems that when radios are brought near monsters, they start to pick up interference generated by them, effectively making the intended communication impossible.
(And on , the cat-tank is shown to have an antenna with a single vertical aerial of about 1 m length. Assuming that it's the standard λ/4 variant, that'ld put the frequency used at roughly 75 MHz, a band used for various air traffic purposes in year 0.)
At the occasion of the first radio contact between HQ and the expedition, it is demonstrated that the most basic form of remote control is in use as well, in that Trond can remotely activate a buzzer in the cat-tank to request that someone picks up the call. It is not known yet how that mechanism's sensitivity to "black speech" interference compares to that of speech transmission.
Long-Distance Communication Edit
Given a proper choice of frequency, radios were a suitable means for literally world-wide communication in pre-Rash times, and an obvious choice for establishing communication links between the post-Rash settlements without the need to send couriers through the Silent World. However, all such long-distance communication that was shown seemed to involve written letters in envelopes, if not personal visits.
There also is no hint so far that the post-Rash Nordics ever used radio broadcasts to try and establish communication with potential other surviving communities.
No reason why the use of long-distance radio communication should be avoided has been given, but if monsters were not only able to send interference but also to sense an active radio transmitter, it would be an obvious disadvantage of such uses.
Other Uses Edit
There is no indication that radio technology is used for any purposes besides communication - e.g., for full-fledged remote control or radar detection.