I just bought my Roku Express+ (3910 series) and the remote has not worked since I got it. I googled everything I could think of and even changed the batteries. I tried to down load the remote app, but I cannot link it without my Roku bing connected to the internet or finding out the IP address. I am not sure if I got a faulty remote or if I am doing something wrong.
For your new remote to operate reliably, make sure that it is pointed directly at the Roku device (it is controlling) and there must be nothing that could block the signal from travelling to the front of that device (where the IR receiver is located).