Customers not able to open invoice on mobile devices

Hi all. I've had so many customers telling me that they can't view their invoices even though I've had the 'Seen' notification come through on my phone, so I've had to save it as a PDF and email it over manually. One customer recently screen recorded themselves trying to open it on their Android device and when they tap the 'View Invoice' button it just doesn't load anything. Today I actually invoiced my mothers company for a job, so I was able to see if she could open it on her Android phone, and it's doing the same thing. I've just sent a test invoice to my private email address to see if I can open it on my new iPhone, and I can't. How can it be that absolutely no-one can view an invoice on mobile?

What's worse, is that when they open the invoice on a computer, the format is all over the place, everything is enlarged and not in line where it should be. I'm using Quickbooks Self Employed which has no settings to change regarding the format or template of invoices. Surely I'm not the only one having this issue? Is there a fix?

TIA!