Tabbing/WCAG Accessibility

I have been working on two accessible courses and while I'm having no issues tabbing/adjusting the tab order as needed on the course slides themselves, I cannot seem to tab IN to the content from an LMS. For example, if I load my file to scormcloud, I can tab into the course and out of the course to the browser address bar ONLY if I'm using Chrome. If using Firefox, IE, or Edge, the course will load and I can tab within the course, but once I cycle out to the browser address bar, I can't get back into the course. This is also happening in a different LMS, not just scormcloud. Has anyone else experienced this? Tabbing is a basic necessity for WCAG and this seems to render storyline useless if I can only offer my course in Chrome. Any advice you can offer is greatly appreciated!

Additional information as needed:

My next and back buttons are custom. I am using the "classic" player (which doesn't really matter since all the controls are "off" and its just operating as a container) and I have a custom menu built (this was another work-around due to not being able to tab out of the course AT ALL to start, even in Chrome... but for whatever reason when I combined a custom menu with the classic player, I could tab out). 

Thanks!

Alana