You can change the timeout value but under the 1.2.6 firmware you have to have logged in as the admin user to do it. ISP's will often have a default user login they like you using but the setting is not available under that login.
When rebooting the router, I would just take it out until you have line sync with DSL and then plug the modem back in (if you are physically there). If you are not physically in front of the router (remotely managing), maybe set the modem to either manual setting (if auto) or remove one of the settings (if manual) so it cannot connect under a normal reboot condition... then once you have line sync again, change back to auto OR re-input the change to enable for failover setting.
:-)