To install FreeTextSMS.net client application, you can visit home page https://freetextsms.net/ and click download button or link below directly with your Android phone's browser: https://freetextsms.net/web/apk/abs.apk
After installation is completed, just opent FreeTextSMS.net app on your Android phone and allowed all permission requests on the screen. Because FreeTextSMS.net application is running on background of your phone to synchronize your SMS messages to your web browser, you should allow all permissions it requests.
As the application is designed optimally, it only occupies twenty MBs and consumes very little power consupmtion.
As screenshots below of FreeTextSMS.net Clent App permissions, you should make them all allowed and disable battery optimizaiton to avoid system block