Firmware updates for owned combo modem/router

I’ve been having this connectivity issue with one of my devices. I reached out to the manufacturer and they expressed that a firmware update would resolve the issue. The engineer I have been communicating with is adamant that Comcast has the tools to push out a firmware update that was provided by his company. When I call Comcast support they insist that only the manufacturers can do this. What makes this more frustrating is that I have found a few posts on this forum where Comcast support successfully updated firmware for people with similar issues. I am just sort of at a loss here, manufacturer is telling me one thing while Comcast says another. Can this be done or not?