FoundryVTT. It’s a one time license fee and though the code is not completely open source, it’s mostly just client side JavaScript that is completely unobfuscated and supposed to be able to be viewed so everyone can modify it with addons. The server is obfuscated JavaScript.
It’s completely and easily self-hostable.
So far, the company/people behind are just TTRPG nerds just wanting a good and friendly VTT (…wanting to make money with it).
It is a full VTT though. If you really only want chat+dice rolling, I would suggest a matrix server with a dice roll plugin/bot.






The foundry specs are so low you can probably use a toaster for the hosting. The client (web browser) uses graphics for the tabletop so the requirements are a bit higher, but still every device currently available will be able to run it I’m pretty sure.
The main difficult requirement is the networking, whereever you host it will need an open port for people to connect to. I don’t know how hard that will be for you.
Don’t know much about the Matrix stuff, I’ve just seen it before.