Either use NetInstall to reinstall the firmware, and uncheck the 'save configuration' option. Alternatively read http://www.roc-noc.com/pdf/RB1100/rb1100ugA.pdf
, which on page 3 shows the schematic of the board and mentions:
It is possible to reset all software configuration by using the Software Reset 2 button hole ( C229, see
schematic) during the power-up. No confirmation or passwords will be asked, so use with caution. This
feature can be disabled in the “system routerboard settings” menu by switching the “enable-jumperreset” parameter to “no”. Simply use a metal object to short circuit the metallic reset hole (TP12) while
C229 is two half circuits that don't connect, bridge them with a screwdriver during boot and the board will reset.
After all settings have been reset to default, the default username is 'admin', with a blank password.
Specific answers require specific questions. When in doubt, post the output of "/ip address print detail", "/ip route print detail", "/interface print detail", "/ip firewall export", and an accurate network diagram.