mail server blocked after comcast modem replacement

I provide free webhosting and mail service for very small churches with no budget ( and one small, family-owned trucking company).  This is a no-income ministry/hobby. After a recent modem crash and replacement, Comcast switched my modem from Bridge mode to Gateway mode. When using Bridge mode with the old modem, everything worked. With the new modem, I have no internet access with Bridge mode and am told by CS that Bridge Mode doesn't work with an 8 IP subnet.  With the old modem, it worked fine for years.  Using the modem as a Gateway, outgoing mail headers rewrite the IP address of the sending mail server from the mail server's IP to the modem's gateway IP address.  Consequently, my DNS DKIM doesn't match, causing most outgoing mail to bounce.

Help.