No remote and needd to use mobile app but wont let me connect
Not possible if the roku hasn't been set to the same network and password as the roku app. You would need a remote to do that first, but it doesn't have to be true roku remote since the express takes IR remote, so a cheap universal with roku code will work.
As @Tivoburkee mentioned, you will need a remote to set up the device and then you would be able to use the mobile app as a remote.
If you need a replacement remote, you can purchase the Roku Standard IR Remote on our Support page here: Player Accessories
Thanks,
Danny