The macOS version and the iOS/iPadOS version of Textastic are two separate apps for two different platforms with different feature sets: the macOS version doesn't have remote connection support.

I didn't add an FTP client to the Mac version of Textastic since there are so many free and commercial apps for Mac that can do FTP transfers.

Here is a small selection:

Free: Cyberduck, FileZilla
Commercial: Transmit, YummyFTP, ForkLift, PathFinder, Commander One

I use ForkLift myself as a Finder replacement and SFTP Client.

You could even use Transmit to mount an FTP connection in Finder. You can then open files from the mounted folder and edit files live on the FTP server.