Since you forgot the password to your router, you need to reset it to factory defaults:

There are two (2) ways to reset the Linksys EA6900 to factory defaults:

Press and hold the Reset button on the back panel for about 10 seconds then release.

Log in to your Linksys Smart Wi-Fi Account. Under Router Settings, click Troubleshooting > Diagnostics. Click Reset under Factory reset.

• Router username and password.
• Wi-Fi username and password.
• Firewall settings.
• Portforwards you have setup.
• Any other changes you have made.