R8000P Nighthawk X6S AC4000 - Unable connecting internet modem directly to WAN Port

I have a R8000P with the current firmware V1.4.1.24_1.2.20. If I put the 100Mbit ethernet cable from the internet modem of the internet provider into the WAN port of the R8000P, the connectivity light stays out. If I switch the calbe from R8000P to the WAN port of the old WNR2200, the connectivity will be there immediately.

Now, I put the cable into a LAN port of the WNR2200 and connect the WAN port of the R8000P on a other LAN port of WNR2200, the R8000P router immediately gets an official IP address from the internet provider.

Why the connectivity is not possible between the internet modem of the internet provider and the R8000P?

I think, it's a problem of the WAN port, isn't it?

Thanks for the help.