Where to put submit button in a long screen with multiple inputs

I'm redoing a web configuration UI for a piece of hardware. Think of a configuration UI of a standard router. You install this hardware into your network and access configuration through your browser. The UI will normally be accessed from PC and should also be available for mobile devices, but PC is definitely in a focus here.

I need to implement a configuration form with a submit button. The old design was done in material design and had this layout: enter image description here

Form1, Form2 are a bit different configuration forms, that have a similar topic (Currently active SubMenu1). Think of "IP Address", "Router Name", "DHCP Configuration" etc. if it was a router. All of these have the same Submit button in the right top corner that applies only the changed inputs. For example if there is an input for a password, it will be applied only if it was changed, otherwise, the old password stays even though the input was empty. The submit button, although located on a menu bar is floating (when the menu bar is not) and always stays on the screen in exactly the same location when you scroll.

I'm redoing a UI similarly, preserving looks more or less, but changing the forms to be grouped in cards instead:

enter image description here

But I'm hesitant to use the same location for Submit button, as I believe there are serious issues with it from UI/UX perspective.

  1. The location is extremely weird and unintuitive. The button is located on the menu that is common for all views and simply appears/disappears depending on the view. Users do not expect it to be there
  2. The button although located on the menu is not attached to it and floats when menu is not.
  3. It is not obvious that if you do not change the password input and leave it empty, the password is not changed. My first thought is that this submits an empty password to the system. However, I'm not 100% sure this is a problem, just find it weird.

The problem is I do not know where to put submit button here. My first guess it to make it floating and to put it in the lower right corner. I do not know how to fix the password problem though.

Is this a good suggestion? If not what should I do? And is this a good idea to leave the password box as is, or it needs to be changed?

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