@LostinPhilly
Your Express 3900 is a lower end unit that can only connect to a router using the 2.4 GHz WiFi band, on channels 1-11, using b/g/n protocols
Your Asus GT6 may not have its 2.4 GHz WiFi band turned on, or if its on, may be configured to use a 2.4 GHz channel above 11, or protocols other than b/g/n (just g/n is not sufficient).
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X