Internal LAN Problem with C6250 CableModem!

Per the forum's posting request for detailed device info:

Model: C6250-100NAS

Firmware: V1.01.26

Hardware: 1.02

Cable Port: Provisioned @ 16 up channels x 4 down channels

Internet Port: WAN IP's granted and defined.

2.4GHz WiFi: Up Running at 300Mbps

5GHz WiFi: Up Running at 1.3Gbps

All Guest Newroks are DISABLED!

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Hello All,

I have searched the WEB, in general, and here in these forums for a solution to my very strange problem...  I have seen issues of "Invalid Data. Lan IP Address Overlapped!" reported in these forums (I get that error also, depending on the operation I am trying to accomplish!).

I have a C6250 CableModem, the internal LAN segment is set to 192.168.208.XXX and the DHCP capability of the device is disabled and being done by another system (in this instance, a Raspberry Pi running Pi-Hole).

Everything is operational, the CableModem is properly sync'ing with the ISP (after a few hours on both chat and the phone conversations wth the ISP's technical support organizaton!).  All of the devces and systems on my internal network can succesfully access the Internet!

So that's all the GOOD news, my problem is that whenever I try to setup a custom port-forwarding service (in this case OpenVPN) - or even try the DMZ feature -  to a device on the internal LAN, I get the following error message "Invalid Data. Internal IP Address is not in LAN!".  As previously indicated, the C6250 has its IP address set to 192.168.208.1 (and is the subnet's gateway device).  I have tried myriad things to try to get the C6250 to accept the forwarding, to no avail.  Nothing I do avoids getting the above failure message!

In fact, I can't even change the IP address of the cable modem.  For example, I tried to set the C6250's IP address to 192.168.0.1, I then get the following error "Invalid Data. Lan IP Address Overlapped!"!  I have seen mention that the Guest Network support in these devices seem to reserve a blocks of address and that could be reason for these errors messages.  

Also, another strage behavior seen is that a device that connects to the 2.4G WiFi network (NOT the Guest Network!) that happens to have a static IP of 192.168.208.20 sometimes shows up in the "Attached Devices" page with an IP addres on the segment 192.168.1.ZZZ (where "ZZZ" is a random number!!!!).  Occasionally, the device listing hows the device with the expected MAC address having the correct IP address of 192.168.208.20...  I have no clue what that is about....  By the way, regardless of what the C6250 shows in the attached devices page, the 192.168.208.20 device is alway ping'able using the C6250's diagnostic testing page....

My question is: Has anyone else seen this problem and is there a fix for this?  Or do I need to turn off the router features of this CableModem and just run in bridged-mode and go purchase another device to be the NAT'ing Firewall and WiFi AP correctly/properly?  After the trouple of getting the CableModem piece working I don't want to go down that rat-hole again getting another device provsioned!!!!

Thanks, in advance, for any assistance you all can provide!!!!!!!!