I don’t use a dedicated app at all to use Lemmy on my Librem 5 daily-driver. I access it via Firefox. To do this, I created a dedicated Firefox profile for Lemmy and created an app icon for it in the app gallery. More details here.

While not a physical radio, a Linux phone such as the Librem 5 in conjunction with an RTL-SDR dongle and external antenna may be a good candidate for a mobile software-defined radio (SDR) transceiver.

SDR frameworks such as GNUradio or REDHAWK are well-established by this point. Newer versions of REDHAWK are designed to run on CentOS/Rocky Linux, however, and they don’t (AFAIK) come with a mobile-friendly UI.

I do know that there are some web-based SDR tools in the wild. I’m not very familiar with them, their system requirements/capabilities/limitations, but they could be worth a look to jump-start a Progressive Web App for mobile devices.