The post on the form here suggests setting the receiver to (4) Mode-s Beast and this works for me.
https://forum.flightradar24.com/threads/10232-How-to-Install-dump1090-mutability_1-15-dev-on-RPi
The above instruction are for those who have installed dump1090-mutability v1.15~dev and therefore will NOT use FR24 supplied dump1090-mutability v1.14.
In case of @Millsey, he mentioned he is using the FR24 supplied dump1090. The settings "receiver=dvbt" I have shown in my above post are for this situation, as the FR24 supplied dump1090 will now use the DVB-T dongle.
The setting "Beast, 127.0.0.1:30005" are required when the user has installed a decoder software (dump1090) of his own choice, and DVB-T will be used by that software. FR24 feeder will get data from that decoder software on port 30005, format beast.
Is it better to select (1) DVBT USB) to get other options via procargs?
If you select DVBT (USB), FR24 feeder will start its own dump1090 at startup and grab the DVBT. Your dump1090-mutability will become orphan - no dongle left for it.
The FR24 feeder starts/stops and passes these arguments at startup ONLY to the dump1090 provided by FR24.
The FR24 feeder has no control over user installed dump1090, and does NOT start/stop it or pass any process arguments to it.
The FR24feeder in this case only "listens" to the data from user installed dump1090.
User has to make configuration settings of his dump1090 by editing the file /etc/default/dump1090-mutability.