TextNow, like most modern apps, verifies account and credit balances server-side, not on the user's device. A "mod" cannot legally or technically generate unlimited server-side credits, making these claims fraudulent.
It is highly recommended to downloading, installing, or using modded APK files from unofficial sources. Risks of Using Modded APKs
Modified APKs, especially those promising "unlimited credits," are primary carriers for malware, spyware, and ransomware, which can steal personal data or hijack your device.