Yes it is possible, but Android phones and Chromebooks are a bit hit and miss as to whether it works.
Personally I have found that using a cheap miracast standard dongle in a different HDMI port to be more effective for casting than Roku. Should cost no more than the express for a cheap noname one, bit more for a Microsoft branded one.
Or a certain other company's casting/streaming device. You know, the company that owns the Android platform....
There is casting, and there is mirroring, and they are not the same, though sometimes they are named the same.
Its not clear whether you are trying to cast (from an app) or mirror the display (from the OS)
Roku supports Miracast (mirroring), DIAL (casting), AirPlay2 (casting & mirroring) and PlayOnRoku (casting).
Your Motorola G6 is Android based, and unfortunately Google deprecated/removed Miracast support in favor of GoogleCast/Chromecast (casting & mirroring) in more recent Android versions, though some OEMs add it back (e.g. Samsung).
You'll need to look for a Miracast-compatible/capable app in the Play store if you want to mirror your phone's display to your Roku.
Some DIAL-enabled apps (NF/YT) will be able to cast to your Roku since Roku has DIAL support.
Otherwise, you'll need to get a GoogleCast/ChromeCast-capable Android TV-based device to mirror or cast from your phone (e.g. GCCWGTV/O4KSB/TS4K/etc).
Again, If you want to mirror to your Roku from your Android phone, you'll need Miracast capability - either via the OS or an app. Your G6 doesnt have it via the OS, and you havent found an app that enables it.
Sans Miracast capability on your phone, you'll need to get a streamer with ChromeCast/GoogleCast capability - do NOT get the ONN FHD dongle; its only 1080p, and for $5 more you can get the ONN 4K Streaming Box, and you'll have all the casting & mirroring capability/compatability you want for your phone.