You can try to enable the alternative service if your messages are staying in Pending status for a long time. You just have to open the app and click on the Play button to start the service as highlighted in the below screenshot.
It is still a good idea to follow the below instructions.
Note for phones without Stock Android
Some device manufacturers like Mi, Honor, and some others use their own battery saver app. You will need to add the SMS Gateway app into the exceptions list in the battery saver app if your phone is using a custom battery saver. For example, you can see how I added the SMS Gateway app into exceptions list on Redimi Note 3.
If you are using a device that doesn't have the stock Android then it may have AutoStart permission settings. You need to download the latest version of the app from here. If after installing, it prompts you with the message "Your device is restricting the application's functionality. Please enable/add the application in the whitelist so it can work normally." then it means it is restricting the app. Just press the OK button and it will take you to the screen where you can whitelist your app.
There is a chance that the latest version of the app can't detect the AutoStart permission cause it is not possible to test the app on every device. In that case, you should consult your device manufacturer to see if there is any setting like that available on your device.
If you are using the Xiaomi phone then make sure you granted AutoStart permission to the SMS Gateway app as shown in the following video.
If you are using the device with Funtouch OS try the instructions shown here.
Even after following the above instructions if you still encounter the issue then you need to try the app on a different device. It is recommended that you use a Stock Android device or Samsung device for the best experience. It is also preferable to use a device with 4 GB or more RAM for the best experience.