Sense 2 GPS map is inaccurate

[NZCJS]

I have just purchased two FitBits. One, a Charge 6, the other a Sense 2.  My wife uses the Charge 6 and I wanted a bigger screen, so use the Sense 2.

When we go for walks the Charge 6 is faultless with maps. The Sense 2 is appalling. Either I get nothing at all or the map looks like a two year old has scribbled all over it. The altimeter also is unreliable and doesn't always record heights.

I have shutdown, reset to factory, deleted the Sense 2 from the Fitbit app, removed the Bluetooth settings for the Sense 2 and started again. The problem persists.

I understood the Sense 2 had built in GPS and there was no need to take a phone with you.  Without a phone nearby by Sense 2 just reports "GPS Connecting" but it never does.

Any suggestions? Please!!

Moderator Edit: Clarified subject

[NZCJS]

I contacted FitBit support and detailed the issues. I replaced my Sense 2 and the replacement is working faultlessly. GPS and maps work well without the need of my phone. Long button press to start exercise and get maps is fine also.  All on all this new Sense 2 is working as it should.