Also make sure in the Samsung Email or Gmail app settings for which ever one you're using you go to Mobile Settings and have both allow background data and allow background data even when data saver is turned on set to on.
ℹ️ Email or Gmail
ℹ️ Mobile Data
Go to settings>tap on magnifying glass> and type Optimize, then select Optimize battery usage. Once there, tap on Apps not optimized and select All. From the list find your Gmail app and if it is optimized, then turn it off