I am also finding this very frustrating, but oh so typical of Shaw and the crap Arris hardware / firmware that Shaw seems to love for some reason.
Like others, I find the ability to configure the XB6 so far behind routers that are 10+ years older in hardware / firmware. There is also this push to internet.shaw.ca which just provides almost nothing, and is so latent with respect to the current status of the XB6. I have added / removed devices that show up on the local LAN management interface right away (10.0.0.1 or similar), but can take so long to be reflected at internet.shaw.ca. This might be useful if you need to debug remotely, but is so severely lacking in functionality that is almost useless. Same comment applies to the analogous Blue Curve app.
Like others, seems like the best thing might be to put the XB6 into bridge mode.
Its Jan 2022 and the same problem is on the XB7. In my case I was trying to reserve my wifi printer's fixed IP. I got into a mess because the xb7 auto assigned the name and mac and IP with the wrong details and now I can't delete or change it without a hardware reset. Guess I'll use my ten year old netgear to connect to my printer. Very disappointing that I'd have to do that.
@UserGH -- do you always leave your printer powered-on? If so, and if you have enabled DHCP (Dynamic Host Configuration Protocol) on the printer, then the printer will keep using the same IP-address forever - "renewing" it occasionally. Also, the cable-modem "remembers" the MAC-address of the printer. So, if you power-off your printer for a long time (a week?), and then power-on your printer, the cable-modem will "remember" the IP-address that it previously gave to the printer's MAC-address, and will supply the previous IP-address.
So, I see little need to create a "static" IP-address for the printer -- after the first time that it connects to the cable-modem, it will keep the same IP-address. This is a nice feature.
I had a similar problem recently. My issue was that I couldn't manually choose what ip to reserve. The reservation seemed to go through but with the same ip it had previously during DHCP.
My fix was to add a note during the reservation. This seemed to solve it for me