iPhone:~# wget http://dl-cdn.alpinelinux.org/alpine/latest-stable/main/x86/apk-tools-static-2.10.5-r1.apk wget: bad address 'dl-cdn.alpinelinux.org'
mount -t ios . /mnt # Pick "Downloads" from "On My iPad" in the GUI
tar xvzf /mnt/apk-tools-static-2.10.5-r1.apk
Though, I suspect apk itself will fail if you can't resolve domain names.
Edit: As gorkish points out, make sure iSH has network permissions first and try again with the original command.