On openwrt, the peers must be defined as /32.
OK, I changed in both peers the IP address into /32
I also changed this in openwrt-> Interfaces » wireguard into /32
Restarted the network via /etc/init.d/network restart
Now, everything is working.
@psherman @egc @mk24 thanks for all your replies, together you made it working!
I will now review all the configurations, and try to understand why it works now. I will mark the post that solved the biggest problem as solution, but thanks goes to your all!!
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Thanks! 
For everyone struggling with the same situation I described everything I did to make this work (with the reactions of everyone in this thread) on my blog page, https://basaandewiel.github.io/Wireguard_on_openwrt_behind_isp_router/
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