Python nor pip does install rrdtool

pip and source installation of rrdtool does not work on my rpi3b+.
How can i find out more in depth what the failure is?

Like the error-message there says, you are missing rrdtool's header-files and/or libraries. python-rrdtool != rrdtool.

Do you mean I have to install the packages itself?
I have installed them over opkg.

python-rrdtool is just python-bindings for rrdtool and pip has to compile those bindings, but it can't compile the bindings without the header-files, which is your problem.

Alas, the OpenWrt-packages do not include header-files, since OpenWrt is meant for resource-constrained systems like e.g. routers and header-files are only needed when compiling software -- there is no point in compiling software on routers or such and header-files would just take space for no good reason! This means that you'll have to either recompile the rrdtool-package with the OpenWrt SDK and copy the header-files to your system or you'll need to properly package the python-rrdtool for OpenWrt yourself.

I also have to point out that you have chosen the wrong OS for this stuff. As they say, use the right tool for the job: OpenWrt for routing, firewalling etc. and a proper Linux-distro for general development-tasks.

Might be true. But name me a distro that comes with a webfrontend and a community this great.
I only want to plot some temperature charts, but access is a big issue when I use ubuntu and grafana... I checked fairly enough possibilities. A Raspery Pi with WLAN AP, DHCP and separate LAN non DHCP is realy nothing easy with other distros. OpenWRT has some really nice services.
Also, uci is something so nice...

My first attempt seams more fisible as collectd-mod-onewire should do exactly what I need. Just need to find out how to activate that package.