I observe that threat and must say that upgrade must be done only via MikroTik Support.
Please write to
suppport@mikrotik.com or create account at
https://help.mikrotik.com/servicedesk . You should have a case already, try find it and send answer at exist ticket.
This modem can be upgraded from FILE but I thinks it's not be that easy if you use the "beta/release candicate" firmware's.
In my situation, all test firmware was installed by MikroTik support only.
If you not have the Public IP then probably the MikroTik ask you to create a vpn to their VPN Serwer and that way MikroTik Support can do support at your hardware via internal ssh/winbox etc.
They not use any Remote Access apps like TeamViewer etc - I think.
This should be te same way in with you install your current firmware.
Give your feedback later.