Request Finance is compatible with all wallets, including wallets that support WalletConnect.
Please use the Chrome mobile app to log in to Request Finance. Once you are ready to pay an invoice, click on the "Pay Now" button and then select "WalletConnect." Your mobile should then offer to pay with any compatible app installed on your device, including the Metamask mobile app, Trust Wallet, etc.
Note about Metamask's mobile app: Please do not use Metamask mobile's internal browser. Always prefer Chrome to browse Request Finance. Metamask's internal browser suffers from an underlying issue that renders it incompatible with Request Finance.