What about VPN? If you don’t care if a third party is involved, as long as the traffic is encrypted, I can recommend using Tailscale. Install the client on the target and your system and just copy files as you would be in the same LAN by using their 100.x.x.x adresses.
If you want to be totally independent of other vendors, you can setup wireguard (also used internally by tailscale) and connect your clients.
If you can’t open ingress ports on any of your sides and you don’t want to use tailscale, you can still spun up a cheap VPS, install wireguard there and connect your clients system and your client both via VPN to this system. Afterwards, you can copy files via the internal wireguard IPs.
But, if you look for a simple solution, tailscale is my preferred way.
You could even use taildrop to transfer files
https://tailscale.com/kb/1106/taildrop/
What about VPN? If you don’t care if a third party is involved, as long as the traffic is encrypted, I can recommend using Tailscale. Install the client on the target and your system and just copy files as you would be in the same LAN by using their 100.x.x.x adresses. If you want to be totally independent of other vendors, you can setup wireguard (also used internally by tailscale) and connect your clients.
If you can’t open ingress ports on any of your sides and you don’t want to use tailscale, you can still spun up a cheap VPS, install wireguard there and connect your clients system and your client both via VPN to this system. Afterwards, you can copy files via the internal wireguard IPs.
But, if you look for a simple solution, tailscale is my preferred way. You could even use taildrop to transfer files https://tailscale.com/kb/1106/taildrop/