*Proper* Static IP Config

I am asking this here since Comcast Reps will not advise on device configuration for Static IPs., so I am hoping someone can shed some light on this.

I have a BWG modem (being replaced soon) and a block of 5 Static IPs, x.x.x.25 – x.x.x.29, with a "gateway IP" of x.x.x.30.

Now, if I manually configure my ethernet-connected server machine to have any one (or multiple) of the static IPs (.25-.29), a mask of 255.255.255.248, and a default gateway of x.x.x.30, and no IP address(es) that fall within the local subnet of the modem (10.x.x.x), the internet for that server is "broken" and I cannot browse the web from the machine at all.  Responses to inbound requests solicited from outside the network are horrifically slow or fail altogether.

But, if I configure the server with a primary IP address that falls within the modem's local subnet (say 10.x.x.101), a mask of 255.255.255.0, and a gateway IP of 10.x.x.1, and then add a Comcast static as a second IP address (say x.x.x.25) with mask of 255.255.255.248, then browsing works on the machine, and responses from the server in answer to inbound requests travel at expected speed and don't fail.

So then the server PC has the following setup:

IP: 10.x.x.101

Mask: 255.255.255.0

Gateway: 10.x.x.1

IP: x.x.x.25

Mask: 255.255.255.248

Note the conspicuous lack of the x.x.x.30 default gateway.  If I add it in as a "second" default gateway to go with the Comcast static IP that supposedly "belongs" to it, it has the effect of intermittently destroying the internet access, just like when it was the only gateway, except now only part of the time.

This totally defies my understanding of the expected static IP configuration, especially when Comcast's own documentation seems to suggest that in order to use static IPs properly, you're supposed to enter the static gateway (x.x.x.30) as the default gateway on your device, alongside the static IP(s).  But when I do this, it breaks all kinds of everything. The only way to get it working is to have a local subnet IP/Mask/Default Gateway as the primary, and then add the static(s) and mask(s) as alternates, ommitting the Comcast-provided static gateway IP altogether.

What's going on here?  Is this a bug or something that's unique to the BWG?  I never had this issue with the CBR1/2 (temporarily removed due to other issues with static IPs and admin tool login).