Firmware is now available for the ZyXel VMG 1312-B10A which fixes the lack of support for MTU 1508 in modem-only. It just works, fine. A pre-built bin file is available and the complete sources are I'm told on github and I have read the build instructions, but have not built it from scratch myself. One or two others that I know of are using it. It was built by someone who had got fed up with ZyXelís inertia / donít-care attitude.
Would AA be willing to evaluate it?