Roku Express won't connect to the wifi any more, but it works on my phone's hotspot

Thank you again, renojim and atc98092 for the responses. After looking into your good advice, plus long calls with both my ISP and Roku support during which I promised I had tried everything their scripts were telling them to tell me, I felt like my options were spent. So I tried once again rebooting my modem and router and -- third time was the charm -- everything started working as desired. But I certainly learned a lot about other details along the way!