I saw the other comment on Reddit about searching pg. 486 of the General Lexus Manual for a 2019 Lexus RX. My family owns a newer RX, and this has similar features. I suspect that the setting for default city is tied to the home location that is set up in the Navigation System.
I took a quick peek at the 2019 Lexus RX Navigation & Multimedia Systems Manual. The Lexus manuals are free to download for Lexus models at:
https://www.lexus.com/drivers/resources
In the Nav & MMedia manual, it starts on pg. 22 to setting up your home location, then refers to pg. 91 and pg. 121 to set the Home Location.
Pg. 121 has the best applicable instructions, under the Title: SETTING UP HOME.
1. Goto to Navigation Settings
2. Select HOME
3. Select one of 3 choices to set your Home Location:
[1] REGISTERING HOME - Select to Register Home.
[2] EDITING HOME - Select to Edit Home.
[3] DELETING HOME - Select to Delete Home.
If I had the same problem, I would try and DELETE HOME first, the do the REGISTER HOME next. Heck, I would even do the EDIT HOME to see if what I registered was correct.
I suspect the HOME LOCATION is tied to the default location in WEATHER. Wouldn’t hurt to try, because this is a lot easier than taking battery power off and then restoring it (which has not worked, already).
Best of Luck! Let us know how it goes..