Assuming you know your Mom's Wi-Fi SSID and password, I would change my phone's hotspot SSID to her Wi-Fi SSID and its password to the same as her Wi-Fi password. Then I'd connect to it with the Roku and save it, then power down. It should automatically connect when powered up at your Mom's.
Alternatively, I'd copy and paste my router SSID and password into a text file, make the changes to the router, connect to it and save in the Roku, power down the Roku, then paste the original info back into the router.