UPDATE:
I installed apt install conntrack as suggested. I'm not sure if the kernel is correctly setup, which is my main question here.
But moving on to the third section, compiling it fails too. I wonder if this is an issue on the repo or the minimum requirements aren't met.
admin@de-vpn-1:~/dd-wrt/src/router/ndpi-netfilter$ ./autogen.sh autoreconf: Entering directory `.' autoreconf: configure.ac: not using Gettext autoreconf: running: aclocal --force -I m4 autoreconf: configure.ac: tracing autoreconf: running: libtoolize --copy --force libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, '.'. libtoolize: copying file './ltmain.sh' libtoolize: putting macros in AC_CONFIG_MACRO_DIRS, 'm4'. libtoolize: copying file 'm4/libtool.m4' libtoolize: copying file 'm4/ltoptions.m4' libtoolize: copying file 'm4/ltsugar.m4' libtoolize: copying file 'm4/ltversion.m4' libtoolize: copying file 'm4/lt~obsolete.m4' autoreconf: running: /usr/bin/autoconf --force autoreconf: running: /usr/bin/autoheader --force autoreconf: running: automake --add-missing --copy --force-missing configure.ac:54: installing './compile' configure.ac:6: installing './missing' configure.ac:336: error: required file 'example/Makefile.in' not found configure.ac:336: error: required file 'tests/Makefile.in' not found configure.ac:336: error: required file 'tests/unit/Makefile.in' not found configure.ac:336: error: required file 'tests/performance/Makefile.in' not found configure.ac:336: error: required file 'tests/dga/Makefile.in' not found configure.ac:336: error: required file 'rrdtool/Makefile.in' not found configure.ac:336: error: required file 'influxdb/Makefile.in' not found configure.ac:337: error: required file 'tests/do.sh.in' not found Makefile.am:2: error: required directory ./tests does not exist