On Zyxel NBG2105 V1.00(AAGU.2)C0 devices, setting the login cookie to 1 provides administrator access.
https://www.zyxel.com/us/en/support/security_advisories.shtml
https://www.zyxel.com/support/SupportLandingSR.shtml?c=gb&l=en&kbid=M-01490&md=NBG2105