Why would a SEP license forbid publishing modem firmware? The IP[0] involved with the modem firmware is copyrighted and trade secret material; publishing the firmware does not (and cannot) grant a patent license to those licensed standards-essential patents. It only imperils your implementation of the modem firmware, not the whole patent spec.
To wit, Cisco publishes an H.264 codec with BSD-licensed code, even though H.264 is under a shitload of patents that they are licensing. OpenH264 does ship with an additional patent sublicense that applies if you are using their build and installing it in a specific way, but that is particular to MPEG-LA's licensing structure[1]. Publishing an implementation of a patent you are licensing does not automatically sublicense the patent.
[0] Laws that grant the ability to dictate the conduct of your competitors
[1] Cisco is abusing(?) the MPEG-LA royalty fee cap to shield FOSS entities from having to take a license, basically, by paying for them.