currently (03/2022) trying to program plug and play firmware (PnP) on -C or -LX is not supported and any trials are voiding warranty, since the device might not boot anymore.
While the modules are pin compatible they might have different flash chips/or size not supported by PnP.
If -S(-UPW) customers are buying -C or -LX modules instead, they are just putting additional material shortage for the -C and -LX customers.
Multiple customers are asking for solutions to work around the supply chain shortage and Digi product management is discussing it how to distribute the available hardware best to all customers.
For now there is no solution, unless you are a NET+OS (-C) or Linux (-LX) expert being able to program the features of the PnP firmware you need, into your existing custom NET+OS or Linux firmware.
Maybe someone has done so, and implemented a serial to RealPort only NET+OS firmware image which they want to share with the public here?
When using serial port profile TCP sockets, for sure you would need some different NET+OS or Linux firmware.
Be aware that the PnP firmware has lot's of features which were developed and improved over 15 years. Reimplementing them in Linux or NET+OS will be lots of effort if you don't select a very limited sub feature set.
Be aware that there are different PnP firmware variants (82000856 Fusion stack) and some Treck stack version. These stacks might not behave the same like a Linux TCP stack and some applications might not work with the new stack.
Also to consider are the unique password requirements introduced by SB-327 and more security requirements: https://www.digi.com/support/knowledge-base/unique-default-passwords-for-digi-products. When customers are programming PnP on -C or -LX products, customers would be legal responsible to ensure similar security by their programming process. The unique password programmed by Digi on -S-UPW modules can't be erased by a factory reset.