I too have experienced the installation problem of Debian 9.2.1 on Hercules ending with the last visible message on the Hercules console log being "Configure the network device".
Reading the assumption of a possible Hercules bug in this thread, I verified the Hercules source ("service.c"), and added some extra debugging code. The result of this (bit more than trivial) exercise is that I cannot find any problems on the Hercules side of things. The fact that Debian 8.9.0 does this correctly also reinforces my suspicion that this is probably not a Hercules bug, but rather a Debian 9 or Debian 9 installer bug. As a side note, my successful circumvention to obtain a working Debian 9.2.1 s390x installation was to perform it under VM under Hercules. The final result IPL's just fine under Hercules without VM, using Hercules Spinhawk 3.13 or Hercules 4.0 Hyperion, both the official Hyperion as well as the SDL Hyperion by "Fish". I am now in the process of (a) trying to find out why the CTCM device does not always come on-line by itself, sometimes I need to "shutdown -r now" a few times on the Hercules console, (b) learning how to do a persistent addition of an LCS device, (c) repeating the same installation with a QETH device under Hyperion, which works, but also not always comes on-line by itself, and if it does, ignores the static IP address setup but goes DHCP instead. I might need to ask for support later, right now I'm still in the learning phase of things. Thanks, Peter J. Jansen