You can find ADTRAN's security advisories here: Security Advisories
Specifically, the advisory for Spectre and Meltdown can be found here: ADTRAN Spectre and Meltdown Attack Advisory (ADTSA-2018001 REV C)
Any info for the MX2800? They weren't listed on the PDF. Are they vulnerable for Spectre / Meltdown and is there a fix?