If you don’t want to use Nightscout (mine is actually bridged from the Dexcom server), then you could use Nightwatch. With that app, you can link your Dexcom to it and create custom alarms at custom ranges. You could then pair it with an external speaker for added volume if needed. Nightwatch is specifically useful if you have an Android device and/or Android watch.
I know there are other apps out there, but I don’t have first-hand knowledge of them. I highly recommend getting a Nightscout site–as it is FREE and you have the ability to use those alarms to create IFTTT recipes. You can also set different alarm thresholds as a sort of backup to your primary alarms with the receiver.
In my opinion, no system is 100% without fault–so the more layers you have as backup alarms–the better.