@NitroGeorge 

Configure Deco mesh to run in Access Point mode, and instead of two networks you'll have one (combined). Then, you can connect printer to either router, or Deco - both diagrams can do. Printer will be visible for every device connected to router and for every device connected to Deco mesh.

For steps, see Switching a Deco to Access Point (AP) Mode

  @NitroGeorge 

You did ask for simple to implement solution. Access Point mode is the simplest. Here is another option:

1. Turn off MikroTik WiFi. You have Deco mesh, it will provide WiFi coverage;

2. Do not connect anything other than Main Deco to MikroTik Ethernet ports. Have only Main Deco hardwired to MikroTik;

3. Connect printer to Deco mesh, it will be first diagram. If you plan to hardwire printer, you can connect it to any available Ethernet port of any Deco node, Main or Satellite.

Because all devices are now connected to Deco mesh, wired and wireless, they all on the same network and should all see printer. In addition, you can use all Deco mesh features you now have.

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For step 1, if you absolutely can't control any MikroTik settings including WiFi, this is what you can do instead:

1. Configure Deco WiFi mesh (Main Network) with different SSID than MikroTik WiFi. Make all devices Forget MikroTik SSID. Configure all devices connect to Deco mesh SSID.

Steps 2-3 remain the same.