It's a known issue, as you can read in older threads. The service requires HTTP/2, which RouterOS doesn't support yet for DoH.
Recent RouterOS versions have added support for HTTP/2, but currently only for the /tool fetch command, and only for the ARM64 and x86/CHR architecture. You'll have to wait for future versions to also have HTTP/2 support for DoH. However, the support would probably still be limited to the ARM64 and x86/CHR platforms due to storage space constraint (other architectures unfortunately have devices with 16MB limited flash storage).
EDIT: Your Chateau 5G R17 ax would be supported because it's ARM64.