Connecting to Roon remotely outside my home using iPhone

Can I connect to the Roon Core remotely outside my home using iPhone to play the music?

There is no roon supported way to do this. Some people use a VPN, you should search the forum for information on those who do so.
Mobile service is on the roon roadmap but has no delivery date.

I run Roon Core on a laptop as I travel a lot, and that makes it portable, so I can access it with headphones on the laptop or iPhone. Probably not what you are hoping for, but the nearest you will get at present.

Me too. Make sure you have plenty of data available.

I would be using it on WiFi, but I use a USB connected copy of my local music library as Tidal on some of the flaky internet services I have to use is a non-starter.

My mother-in-law doesn’t have WIFI.