without a full screenshot, just guessing....
After you unchecked "Show layer boundary" try:
In the menu Select ➤ None if it's grayed, you can pass this one as you have no selection, if selection ➤ you cannot work/paint outside a selection
or/and in the menu Layer ➤ Layer to Image Size it might be a layer smaller than the image thus you cannot paint/work outside of that layer ➤ we need to enlarge that layer by putting the layer to the image size.
If none of it works, please provide a screenshot with the layers' stack/dialog and the tool options dialog (usually below the tools) with the tool and layer you want to use selected
After you unchecked "Show layer boundary" try:
In the menu Select ➤ None if it's grayed, you can pass this one as you have no selection, if selection ➤ you cannot work/paint outside a selection
or/and in the menu Layer ➤ Layer to Image Size it might be a layer smaller than the image thus you cannot paint/work outside of that layer ➤ we need to enlarge that layer by putting the layer to the image size.
If none of it works, please provide a screenshot with the layers' stack/dialog and the tool options dialog (usually below the tools) with the tool and layer you want to use selected