Hi. Problem solved. Took awhile to figure this out but the instructions are kind of out there in pieces...
1. Made sure the antenna arms were pointing up. (Originally had them perpendicular). Roku started to work then.
2. Umplugged router for 120 seconds, then plugged back in.
3. On my PC, Logged on to the extender's tp link webpage by opening google and connecting to the extender. Followed instructions to enter extender password and a few other steps that the site walked me through, and picked highest level of security available (WPA 2).
4. Unplugged the extender for a minute and plugged it back in.
5. Entered extender password on iPhone and other devices as needed.
6. Now working as advertised.
all if this was after I originally paired the extender to the router. I guess if you have an iPhone you may need to reboot the router to have the router reset the IP addresses, etc as the router syncs with the extender so the iPhone can then recognize and connect to the router through the extender.
somehow, other PCs and Roku did not require all these shut down and reboot steps. May have something to do with security settings on iPhone...? Wi-Fi extender name still has "weak security" as WPA 2, but it looks like this level is sufficient according to most sites other than Apple that recommends WPA 3, which I'm not sure is available on the re650 version, or maybe the xfinity router we now have. Will check on this when we replace the router sometime in the distant future...