When installing QNEAT3 using the QGIS Plugin Manager, a limited selection of algorithms (Shortest Path, OD-Matrices and certain Iso-Area algorithms) is available to you out-of-the-box. This is due to some Iso-Area algorithms (Iso-Contours and Iso-Polygons) relying on the matplotlib library that isn't shipped with QGIS3.
In order to activate the Iso-Area as Polygon/Contour algorithms you have to add the matplotlib library to your QGIS-Python installation.
Depending on your operating system you may install matplotlib manually using the following options:
python-qgis -m pip install matplotlibinto the shell
python-qgis -m pip install matplotlibinto the terminal, then hit Enter and confirm installation with 'yes' when prompted
/Library/Frameworks/Python.framework/Versions/3.x/bin/pip3.x install matplotlib(please replace the version number
xaccording to your installation) into the terminal, then hit Enter and confirm installation with 'yes' when prompted
When running a QNEAT3 algorithm it can be useful to enable the processing window to stay open after the algorithm is finished. With this option enabled, you can inspect the algorithms messages and infos that are logged at runtime in the log window. You may set this option under Settings>Options...>Processing>General>Keep dialog open after running an algorithm (tick the checkbox to enable this option).