I am posting a solution here hoping that it will save other from spending the time that I did. Oddly this did not come up easily with a lot of google searches for me, so maybe this post will come up for others in the future.

I have a Archer BE9700 router with firmware 1.0.7 Build 20251027 rel.50074(5553) connected to a Motorola MB8611 Modem with firmware 8611-19.2.18. I wanted to access the modem's configuration page at 192.168.100.1 but could never get it to load. I could ping and traceroute to that address, but never get the web page. The first suggestion on most Google results said to set up a static route. This did not help nor did many of the other suggestions. That was until I finally found a post about a known bug in the Motorola modems that it called the "Martian Filtering" bug. I think this would be better called the Martian Packet Bug. As I understand it, in response to the http request, the Motorola modem sends an ARP packet with a source address of 192.168.0.1 instead of 198.168.100.1. It is called a Martian packet because it seems to come from another world. The TP-Link router very correctly ignores this because 192.168.0.1 is in the default LAN address space, and this packet arrived on the WAN port. So no further communication happens.

The workaround for this Motorola bug is to change the TP-Link Routers LAN address space from the default of 192.168.0.x to something that does not include 192.168.0.1. The easy way to do this is in the TP-Link admin  page ( default 192.168.0.1) go to Advanced->Network->LAN and change the IP address there to 192.168.1.1. This changes the address of the admin page to 192.168.1.1 and moves the whole LAN address space and DHCP pool to 192.168.1.x. 192.168.0.1 is not in this space so the erroneous packet from the Motorola modem is let through and communication proceeds. 

After the appropriate reboots everything seems to be working as expected. Remember from here on out that the TP-Link admin page will be at 192.168.1.1.

I hope this saves others some time and frustration.