Hey there, Jamjamnctzen.
Welcome to the Apple Support Communities! Thanks for posting about the issues you're experiencing with notifications on your iPhone. We'd be glad to see how we can help you out.
You mentioned that you're seeing a red slash through the ringer volume. Does this occur at all times, or is it limited to specific areas where you change the volume? Also, do you have Change with Buttons turned on or off in Settings > Sounds & Haptics? Does changing that make a difference for the volume of your notifications? More info on ways you can Adjust the volume on iPhone can be located here.
Verify all of these settings are correct and make any adjustments that you think may be incorrect and test it out. Let us know what you find out.
Take care.