Apollo projection support for EPSG 29873 is not complete, user can choose EPSG 29873 from data manager, and portal, but Apollo doesn't have correct CSF definition for EPSG 29873.
Follow the following steps to add support for EPSG 29873 to the Apollo 2020
1. Copy the coordinate system definition file (EPSG29873.csf) that is included with these instructions to the following
directory. This path assumes APOLLO was installed to the default location, adjust it if needed:
C:\Program Files\Hexagon\ERDAS APOLLO\tools\RegisterEPSGCodes\bin
2. Run the program RegisterEPSGCodes.exe found in that same directory.
3. Type in the EPSG value to add, in this case it is the number: 29873
4. Click Register
5. Click Restart IIS AppPools
6. Exit the RegisterEPSGCodes program
If there are any problems with this process, then consult the documentation pages shown in above in the section