Assigning reserved IP addresses for printers

I'm trying again under the Equipment topic, since I got no responses in the original area I posted to.  Apologies if this is redundant, but I don't know where to go for help.  I brought in someone who understands routers, but he wasn't able to understand this issue either.

I'm a beginner at networking, but trying to learn, so I can resolve problems with printers going offline on our small Comcast Business network.  This has to be a common problem!  We have the Technicolor cga413com router and two extenders (installed by Comcast).  Reading instructions for assigning a reserved address, it sounds simple.  Log into the router, display connected devices, pick your printer out of the list and modify the settings to have a reserved IP address.  But I don't see my printer in the list of connected devices, despite it being connected to our Wifi network.  What is a likely reason for that?  Because of the extenders?  All the devices in the router list show IP addresses beginning with 10.1.  But looking at the printer settings, the IP address it was assigned begins with 172.16.  Is this because it's being assigned by the extender?  Any advice on where to go that might help me understand this better?