Hi Elijah,
I am terribly sorry to hear about the issues with your brand new iPhone Xs Max!
There is a few things you can try. We can take in a few factors: Has the cell service always been this spotty even with other phones? Is there any cell towers down or maintenance? and last not least, is there a case on the phone, and if there is what kind of case would it be?
If the answer to the cell service and towers is no, and there is no case try one just in case and if there isn't try removing it. If it works better without a case I would find a new case, it would be a phone issue with the case, more of a case material issue.
if none of those work try reseting the network settings.
- Go to Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.
- Go to the carriers store or call them to make sure your account is active and the service speeds aren't being throttled, check that everything is okay with whatever plan you have.
Last options would be is you remove the SIM card and put it back in, put in a new one who contact the store you bought it from, and if it was from an Apple Store return and exchange it and if it wasn't bought from the store I would try backing up the iPhone and restoring it and if that did no good either, take it to your nearest Apple Store or Authorized Apple Service Provider (AASP) and they should be able to help further than what I offered!
Hope this helps anyone else. Chris.
(I forgot to mention, iOS 12.1 will be coming out soon and is planned to have a fix to a lot of issues with it. Thanks!)