Hi,
I have installed a new Invoice Ninja and i have entered my white label key i have changed the .env file to
#options - snappdf / phantom / hosted_ninja
PDF_GENERATOR=hosted_ninja
I have then used the update secret and when i check the Health Check or create a invoice it is still using phantom JS.
You can open ticket on your hosting and ask them to do it for you.
But prepare every info like where your invoiceninja are installed, and where you want them to run the command and what command.
Or maybe you can copy this forum link and sent it to them (Your hosting).
Most of the time, they will do it for you with not so many question asked, and you can also ask them if they can enable terminal for your cpanel account.