Well, Roll20 is probably your best bet. Even with the subscription, however, don't bother with running VTTs on small devices. Character sheets and initiative tracking can be done on phones using lots of tools, but I have a hard time imaging a VTT experience being positive on a small screen.
The main benefit of Roll20 is that it doesn't require installing any software. Players just need a browser and internet connection. The only other fully web-based VTT that comes to mind is Astral VTT. Astral is great if you want to put int the time to set line of sight but Roll20 is far more evolved and can support even on-the-fly games where you use it as white board / digital chessex mat.
The main problem with Roll20 is players need a good internet connection. That's an issue with any VTT, except with Fantasy Grounds, d20pro, and Map Tools, the DM can run two instances on his computer and share the player view via webconference screenshare (Google Hangouts, MS Teams, Zoom, etc.) But I'm assuming you are looking for a full VTT experience with players moving their own tokens, etc.
I'll also say that I've tried VTTs from a variety of countries, including some with poor bandwidth and strong government censorship.
Roll20 would have problems when bandwidth got low, but at least I could access it. Fantasy Grounds and d20pro just won't work in some countries.
During Gary Con, I can to run a VPN on my cell phone, use 3G data paired to my laptop just so I could connect to Fantasy Grounds. It took a long time for the initial loading and maps took a while to load, but at least I could play. But I would never try to run a game like that. I know that some people have run games in Fantasy Grounds from China using a VPN and a tethered phone with 3G, but just using Roll20 would be much easier if you are not already invested in another platform.
I do recommend using something else for voice unless everyone in your group has great internet bandwidth. When playing Roll20 from areas with crappy internet, I'm generally running Discord, sometimes on my phone to ensure a good connection for the battlemap.