It is a bit hard to understand what exactly you have. Clearly you have limited understanding of what you have. Let me first define things.
A modem is a device that converts the carrier signal (DSL or CATV) into an Ethernet protocol signal. A modem does not have router, switch, or wireless access point (wap) capability in and of itself.
A router has a single WAN or Internet (whichever the maker likes to call it) and at least 1 LAN port (although most home grade devices have a 4 port built in network switch technically making it a switch / router). The router accepts or rejects WAN packets if they meet or fail to meet acceptance criteria and routes packets both from WAN to LAN and among LAN devices. The router specifies the LAN subnet and assigns many LAN IP addresses per its configuration.
A network switch is used as a "gathering point" where wired devices connect together. One router port must connect to the switch to provide Internet access, assign IP addresses, etc. A switch does not provide routing capability.
A wireless access point (wap) provides wireless LAN capability. A wap connects to the network switch.
Now for the more confusing parts:
There are modem / routers which combine the modem and routing capability. Some are called ADSL routers or CATV routers as they mate w the ADSL signal from phone company while others are called CATV routers as they mate w the cable tv signal.
There are switch routers which combine switch and router capabilities. Most home grade routers are switch / routers and they have 4 LAN ports.
There are modem / switch / routers which interface w ADSL or CATV signal, provide routing, and provide a 4 port LAN switch in unit.
Finally the wireless access point (wap) can be integrated into a router to make a wireless switch router. The wap can be integrated with modem, router, and switch as well to make an ADSL switch router wireless or CATV switch router wireless device.
I am of the belief you have 2 different devices as follows
The device you call a modem is probably a modem / switch / router. The carrier signal (CATV or ADSL) plugs into it to provide feed and LAN devices plug into the network switch ports. This router provides IP addresses on one subnet to its private side.
The device you call a Wi Fi router is probably a wireless router and its WAN port is plugged into the network switch port of the modem / switch / router. This router provides IP addresses on its private side in a different subnet. Its private side includes wired and wireless.
I suspect the issue is the different subnets and the wireless router is not configured to communicate with the modem / router subnet.
You can do this:
1. Disable DHCP on wireless router.
2. Assign a static LAN IP address to the wireless router that is on the same subnet as is used by modem router. Save the configuration.
3. Plug the feed from modem / router into a LAN not the WAN port of the router.
4. Reboot wireless router, all wired and wireless devices on router and reassign any static IP addresses on what was the wireless router subnet to be on the subnet of the modem / router.
If properly done, the modem / router assigns IP addresses to all LAN devices on same subnet and all will work.
Check your wireless router on how the above is done; ask specific questions in this forum if you need more help.