Let's kick this off!

I work in a company that specializes in remotely configuring devices. This is mainly about telecom handsets/sims and routers.
We have components that plug into the network nodes and listen for subscriber's events and upon that trigger actions such as sending them configurations by binary-SMS, or instructions via normal SMS, or others. This can be used to alert inbound or outbound roamers about tariffs for example.
Another thing I've worked on is something called an EIR: Equipment Identity Register. It's basically a big DB that every operator has with the equipment identifiers of the devices on the network that have rules attached to them. This can be used to blacklist/whitelist/greylist devices and take actions upon their detection. Additionally, multiple mobile operators in the same country can sync up these DB with a national one, or with an international one offered by institutions like the GSMA.
Something else I touch frequently are sim cards. We do remote configurations of their file system and settings via OTA, which can either be specially crafter binary-SMS or an HTTPS-PSK server. This last one is rarely used as it infers that the operator has to buy sim cards with this feature, which are way more costly for them on a large scale. To do that we have to study specifications related to security involved, and to integrate with something called an HLR/HSS, which is the name of the telecom node that stores the ID of the subscriber (IMSI) along with their actual phone number (MSISDN) and the encryption keys used to attach to the network.
I've also dealt with creating what's called sim applets for java cards, which are 99% of sims today. These are normal java software but that use a very small subset of the java language (only has bytes, short, and arrays as types), no garbage collection, and very small memory space (non-volatile and volatile). You can install them remotely on the sim without user interaction. The capabilities we use is mostly for interfacing with the device via a set of predefined commands called STK (sim toolkit). That means sending SMS, displaying popup, ringtones, asking for the location of the device or its current equipment ID, and others.
The last big project I worked on is something called eSIM, a sim card embedded in the device, that allows to install multiple profiles on it. We provide a server where they can be downloaded from and customize the sim profile for operators. The "profile" is the identity of the sim, it's kind of like setting up their OS configurations. As this is an accredited solution (sim cards are the same tech as credit card, UICC), the site needs secure procedures like rooms with security cameras, and encryption using an HSM (hardware security module). This is pretty fun to follow up the new specifications and products in this space. There's a lot of applications.

A lot of these projects are interesting because it gets us in touch with big vendors, from Apple, Google, to groups of mobile operators.
I don't deal with the radio frequency though, but we do some debugging with network traces if needed.
Yet, this is all at the "edge" of tech in this space but isn't really translatable into what's "marketable" today for other software engineer positions.

One thing I also partly dealt with is remote management of routers, something called CWMP or TR69.

I also do some frontend from time to time using VueJS. The languages we use the most are Perl, Java, C++, Javascript, and a deprecating PHP codebase.