iOS Nextcloud App - "The Internet connection appears offline"

I have my own Nextcloud server running on our home network. HTTPS is enabled; I can access it just from my desktop and mobile browsers. However, when using the iOS Nextcloud app, I get this error when plugging in my URL and if I try to use a QR code:

“Connection error
The Internet connection appears to be offline”

I don’t allow any port forwarding from the external Internet, assuming I would just VPN into my home network if I wasn’t at home. Is that mode supported, assuming that’s what the issue is? Any other ideas are welcome! Thanks!