Then do one thing.. after checking (from previous solved topic) email (and save the OTP in tinydb) the randomly generated OTP to the email that he used.. also create one more variable under the user name in firebase as, Status : Inactive
If he entered the saved OTP then change the tag value status : Active
At the time of login if he not done this and skipped mean, nextime while he open the app, just call this tag, if it is Inactive just alert him