The combination "SK706S + PC822" strongly suggests these are ISP-branded units (e.g., from Telmex, Claro, Tigo, or Vivo in Latin America, or specific European ISPs). Retail TP-Link firmware will NOT work on these units.
The latest firmware for SK706S and PC822 often brings: tp sk706s pc822 firmware download
Finding official firmware can be challenging as this is a component board used by various manufacturers. However, technical communities and third-party repositories host compatible software for specific models: The combination "SK706S + PC822" strongly suggests these
A Google search for this firmware yields hundreds of results from blogs like Swstar, Satdl, MyGsm, and various file-hosting sites (Mediafire, Mega). or Vivo in Latin America