A few points to think about. Sincere grovelling apologies if you know all of this already.
You have two networks. Two LANs.
1. One is very small, it only has two things in it, an address for your ZyXEL modem’s admin interface and possibly an address belonging to your router. This LAN is the network cable between your modem and your router.
2. The main LAN, wired and wireless which your router and all PCs and other devices are on.
Now each of these has a range of addresses associated with it. These ranges of addresses might be 256 addresses wide, but it varies. Where these address ranges are in address space is up to you and your router’s setup defines where they are and how big each range is. The two ranges may or may not overlap.
Your router may or may not know how to direct, or ‘route’, packets to the network your modem is in (that ‘network’ being merely the cable between router and modem). Mine had no clue. I had to configure it specially, to tell it where the modem was and what the address range associated with the router-to-modem cable was. My modem lives at 192.168.1.1 and its associated address range is 192.168.1.0 - 192.168.0.255. In that network, the only other thing is the router itself which is at 192.168.1.254.
My main LAN lives at 81.187.xxx.0 - 81.187.xxx.63 (64 addresses wide). On that network my router lives at 81.187.xxx.62. (Addresses have been faked up.)
The two networks, my main one and the tiny one with my modem on it, have address ranges that do not overlap.
My router has three addresses (at least). One on the main LAN, one on the modem cable and one facing the internet. Yours may or may not have one set up on the modem cable. But apart from that your router will have two or three addresses in the same way as mine.
If the address ranges of your two networks do overlap, or are right on top of each other then your router may or may not know how to route packets to the right network, but it will be pretty difficult to explain things to it, because a mention of one address will be ambiguous, that value will not tell the router which network we’re talking about clearly.
I found it really difficult getting my router set up right so that I could talk to the modem’s admin interface. You might be in luck though and it just happens to already be set up just right so that it has the setup and the understanding. That is by no means guaranteed though.