TFTP isn’t going to work for external users, and FTP isn’t secure (unless, as you’ve discovered, you allow only certain IP addresses, and then DHCP causes you to chase your tail).
The good news with FTP and DHCP is that once a client’s IP address changes, they’ll use the last known configuration settings, so IF NOTHING CHANGES you don’t need to load the configuration every time.
And my good friend Google says that setting up FTP(s) and/or HTTPS isn’t as easy as it sounds.
I use FTP to LAN IPs only, so I can’t help you with specific examples of what secure transfers work.