How to enable my custom Third PersonCharacter to walk on walls?

Close, but worse.

The movement component restricts the angle of a slope you can walk on based on Z Up / regualr gravity.

You can try to open up that angle to allow upsidedown movement as well, but it would really be more practical to write your own movement component for this.

Also, fwiw…
Pray - the original one from 2006 - was the first game (which I remeber and am sure of?) that changed gravity on the go.
It was also the first game with Portals, though you couldnt shoot them like in Valv’s innovation.
The later iteration of the same title which Bethesda (who took a few hours off kicking the skyrim dead horse) published (developed by arkane studios) also had some innovative - or cool anyway - local gravitity changes.
Retaining the round planetoid gimmick first introduced by the homonymous title’s predecessor.

This to say, since 2006 theres a bunch of plugins - some more reliable than others - avaliable in marketplace that do exactly this…