Hello,
If you are connected to the router via Wi - Fi, you should have no problem accessing the Web interface as you are connected to the device's LAN. Could you post the message that you receive, when attempting to login/access the web interface? Could you try and see if you are able to login using SSH client such as PuTTy? Also have you tried to factory reset the device, by holding the reset button for 12 to 20 seconds?
Best regards,
Žygimantas