. It is much safer than existing solution on the routers, Yes I know but this changes in original firmware and I'm afraid about my security in network? I don't have guarantee that those changes not...

Original firmware, with added Filement application and a little modifications in the web interface so you are able to pair your router with your account. Is this safe solution? Who is the author of...

What kind modification introduced this firmware in router?

How do you know that is problem with MTU configuration?

Do you have any password or login from ISP? Can you see dongle status in router settings?

What's your OS? Do you have chipset drivers installed?