Hello Jak5,
Thank you for using Apple Support Communities. If you're using your Personal Hotspot on your iPhone, you're going to want to make sure it's properly set up in order to use it. If it's not set up properly, you might not be able to connect other devices to it. Just in case, this resource can help confirm if Personal Hotspot was set up correctly:
How to set up a Personal Hotspot on your iPhone or iPad - Apple Support
We also have this resource that can help troubleshoot Personal Hotspot issues. Just in case, you need to attempt additional steps: If Personal Hotspot is not working on your iPhone or iPad (Wi-Fi + Cellular) - Apple Support
1.) Check that Personal Hotspot is turned on in Settings > Personal Hotspot. If you can't find or turn on Personal Hotspot, check that your wireless carrier enabled it and that your wireless plan supports it.
2.) Restart the iPhone or iPad that provides Personal Hotspot and the other device that needs to connect to Personal Hotspot.
3.) Make sure that you have the latest version of iOS or iPadOS.
4.) On the iPhone or iPad that provides Personal Hotspot, go to Settings > General > Transfer or Reset [Device] > Reset, then tap Reset Network Settings.
If you can't connect to Personal Hotspot with Wi-Fi
On the device that provides Personal Hotspot, go to Settings > Personal Hotspot, then follow these steps:
1.) Make sure that Allow Others to Join is on.
2.) Remember the device's Wi-Fi network name and password that appear on this screen. If you want to change the Wi-Fi network name, learn how to change your device name.
3.) If you're using an iPhone 12 model or later, iPad Pro 11-inch (3rd generation or later), iPad Pro 12.9-inch (5th generation or later), or iPad mini (6th generation), turn on Maximize Compatibility.* Then try connecting your devices again to your Personal Hotspot.
4.) Stay on this screen until you connect your other device to the Wi-Fi network.
On the device that needs to connect to Personal Hotspot, follow these steps:
1.) Turn Wi-Fi off and back on.
2.) Make sure that you connect to the correct Wi-Fi network. The network name is the same as the iOS device that provides Personal Hotspot and the Personal Hotspot icon
might appear next to it.
3.) If you see a password error when you try to join Personal Hotspot, check the Wi-Fi password: On the device that provides Personal Hotspot, go to Settings > Personal Hotspot to see the Wi-Fi password.
If you confirm that only your Roku can't connect to your Personal Hotspot, you might want to follow Roku's public resource on how to troubleshoot this issue: What should I do if I cannot connect to my home network or the internet?
If those steps don't help, consider reaching out to Ruko support for additional help: Roku Support
Thank you for using Apple Support Communities. Best of luck.