Firstly, get some advice from a professional network/IT company.
If you want to do this properly you're going to need a network device/modem/router that provides multiple WAN connectivity. These allow multiple WANs to be connected together for such purposes as failover, increased bandwidth and bandwidth/traffic separation.
The device can then allow you to create a single (or multiple, via VLAN technology) network where you can combine your LAN and wifi and allow routing between the two (or make one, big network) allowing your LAN and wifi devices to talk to each other and use one or both of the Internet connections.
Such devices, off the top of my head (and from previous experience) are DrayTek Vigor multi-WAN routers.
You'll see an example image above (source: https://www.dsl-warehouse.com/images/draytek-vigor-2920vn-connections.jpg) showing WAN1 and WAN2 which are the separate ISP connections (Ethernet so you can use your ISP equipment but plug them into the DrayTek with minimal configuration) and the four gigabit LAN sockets allowing you to have up to four separate LANs or (from what I can recall) up to 10 (or 20) VLANs between the Ethernet and wireless networks.
You can even create multiple wireless networks so you can have, say, a guest and a staff network and they'll be completely safe from one another.