myVesta

Please leave feedback did it solved the problem.

I don't understand your question, please describe problem further?

I never researched about FileManagers, sorry.
I suggest you to simply upload large files via FTP

Maybe this help: https://github.com/myvesta/vesta/issues/99

login to ssh as root
execute: and enter new admin password.

but if you use "VestaCP FileManager", well, in that case better upload file via FTP.

see viewtopic.php?f=18&t=54

so, in pool.d conf files, alter:
upload_max_filesize
post_max_size

You can't access the site or you can't install letsencrypt?
If first thing is a problem, well... login into myVesta as admin, in upper menu click "IP", add new IP, and then switch site to that IP (Web->yourdomain->Edit)

I think it will not cause any problem.

why?
on hetzner.com you have very cheap (and good quality) cloud, for 2.5 EUR monthly, you can easily create instance, run few days, it will cost you 0.1 EUR ($0.1) for few days, enough to test everything you need.