System 7 Pro, the Macintosh Quadra 840AV or 660AV, and Apple Internet Router 3.0 are compatible with each other. The key is to install the software in the correct order. The problem here is the Apple Internet Router 3.0 installs a version of AppleTalk that is not compatible with System 7 Pro. The easiest way to solve the problem is to install the software in this order:
1) Install System 7 Pro
2) Install Apple Internet Router 3.0
3) Install NSI 1.4 or later
Step 3 ensures that AppleTalk 58.1 or later is installed and this is what System 7 Pro requires.