How to stop mobile usage standby from draining my battery? (opens in new tab)

I have an old phone on Android 8\.0 that I no longer use for making calls, but I still use with Wi-Fi for watching online videos\. It has drained 23% of my battery since the last phone charge\. I already have mobile data turned off but its still using a lot of my battery\. Is there a way to stop it without turning on airplane mode because I still want to use Wi-Fi?