Can I use Ethernet to surf and print via WiFi at the same time?

In the net work system preferences, the list order of the services is important as to which one provides Internet. The top most service that has Internet available will be used for Interne. So, if you put ethernet above Wi-Fi in the service order it should connect to Internet via ethernet. However, I don’t know for sure that you’ll still be able to print over the Wi-Fi network but I don’t see why not.

I do not understand the printer being connected to a modem?

Do you have 2 broadband connections in your home?

Or is the ethernet cable from the printer going through the brick wall and connecting to the same modem your router is connecting to?

There is a 'route' command available via Applications -> Utilities -> Terminal that can be used to direct specific IP address ranges to specific network interfaces. You will have to Google to find setup examples.

If you say that connecting to WiFi switches all your traffic to the WiFi, then your Service Order may be out-of-whack.

System Preferences -> Network -> left column -> Gear icon -> Set Service Order

You want your Ethernet interface to be above your WiFi interface.

If your service order is set correctly then the Ethernet interface, when active will always get network routing priority for IP addressing.

Then if you have a separate subnet IP address range for the WiFi network, you can use the 'route' command to direct those to the WiFi interface.

That is to say, the printer subnet cannot be using the same IP address ranges as the Ethernet subnet. So if 192.16.0.* is the Ethernet subnet IP address range, you DO NOT want the printer to be using the same address range.

Controlling the subnet IP address range is done at the router.

Consider using a power line system. It very cheap and takes less than two minutes to install, it is plug and play.

Powerline uses two plugs, one fits in to an electrical socket near your router and the other fits in to an electrical socket near your printer. I'm using a power line adapter just now to use the internet for my iMac and to my printer in another room.

If the printer has an ethernet connection use the ethernet version: https://www.tp-link.com/uk/home-networking/powerline/tl-pa4010-kit/

If the printer has a wi-fi connection use the wifi version: https://www.tp-link.com/uk/home-networking/powerline/tl-wpa4220-kit/

Yes, I do it all the time. I'm connected via Ethernet to my router/model and have my all-in-one Epson connected via WiFi to our LAN.

Just make sure Ethernet is listed first in the System/Netword preference pane:

I concur with Old Toad. My printer uses WiFI and my Mac uses ethernet. I would also suggest that if your printer is having issues connecting to your router perhaps you need to upgrade your router.

So your printer is not connected via WiFI but is connected directly to the modem via cable. That's not the same as wireless connection.

The printer and the Macs have to be on the same LAN.

Since the brick wall is so impenetrable you'll need to get a wire from the Printer to the router like you do with the modem.

Or get a concrete drill and drill another small hole in the wall to accommodate an Ethernet or USB cable.

Since the terms "modem" and "router" can be used interchangeably around the world, to help clear this up ... at least, for me, What is the make & model of your modem. Actually, it wouldn't hurt to know the same for your router.

The reason I'm asking about the modem is that normally only a single device can be connected to its Ethernet port and modems typically do not provide wireless or routing services. Again, in some countries, modem = router, but in the U.S., they are not the same as a router.

If one connect the wifi router to both wired and wifi devices and its set to bridge the networks it should, if you connect Ethernet directly to the modem them it depend on the modem

If the modem has printer support and WiFi support, then it is a modem/WiFi Router combination unit.

You need to put the router on the other side of the brick wall into Bridge mode (or it might be you have to turn off the DHCP server, or NAT server, or some setting that basically turn off routing and allows the modem/WiFi Router to be the main router in the house.

Then all the Macs and the printer will be on the same LAN, and everyone should be able to see everyone else.

It would also be good to give WiFi A and WiFi B the same SSID (WiFi name) and the same password, then you have a roaming network.

The most important thing is getting all the devices on the same subnet, because if I am guessing correctly, you have 2 subnets, and that makes it next to impossible for the Macs to see the printer as most printer protocols do not cross a subnet boundary, and that 2nd router is a subnet boundary as long as it is acting as a router.