I've finally connected my Mitra instance to a real blockchain and configured the subscription service. The blockchain is Polygon PoS which is EVM-compatible #Ethereum sidechain. It is fast and cheap - ideal for payments. Of course this comes at the cost of decentralization so it's not a good choice for storing large amounts of money.
This is my subscription page: https://mitra.social/profile/dd4ebc18-269d-4c7b-a310-03d29c6ab551/subscription. It accepts payments in WMATIC token which is a ERC-20 version of MATIC, the native currency of Polygon blockchain.
Yesterday I successfully started a payment stream from my account on another Mitra instance. You can try that too. The server will recognize your payment if you have ethereum address in your profile (in the $ETH field). Then you will be able to see my subscriber-only posts, though I don't think I will use paywall for anything other than testing this feature.
The contracts can be found here: https://codeberg.org/silverpill/mitra-contracts (I'm using AdapterMini variant).