Latest version working on RPI and other SBC should be MongoDB 4.4
The inst guide is working 1:1 on Debian.
This has nothing to do with DietPi. It’s a limitation of MongoDB not being able to run on SBC’s since version MongoDB 5 and above.
EDIT:
looks like even latest version of 4.4.26 is not able to run on ARM anymore. Means, you need to install older version like 4.4.9 and pin MongoDB on this version or try Docker images
apt-get install -y mongodb-org=4.4.9 mongodb-org-server=4.4.9 mongodb-org-shell=4.4.9 mongodb-org-mongos=4.4.9 mongodb-org-tools=4.4.9