iPhone max slow WiFi/LTE

Did you really tried everything?

These are the basic troubleshooting steps I can suggest:

Force restart your device: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the Apple logo.

If this doesn't work, install a fresh copy of iOS 12. To perform this operation, make a full backup of your XS Max, then restore it to factory settings, and at the end, set it up as new:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

When done, test it for a while, WITHOUT restoring data from your previously backup. If it still is very slow, have it inspected at your most convenient Apple store or Authorized Service Provider: Apple - Find Locations - Location Selector

If, on the contrary, the speed is normal and everything is working fine, you can exclude a hardware defect.

Then restore data from your previously made backup.

If the defect comes back again, your data could be corrupted.

Regards and best of luck.

Giulio

Also be sure to check for a carrier update.

Update your carrier settings on your iPhone or iPad - Apple Support