@Naz. 

To be the deliverer of bad news, this is NOT a Deco issue and rather the printer itself.   Come accross this dozens of times with Mesh systems from all vendors.

In short most printers have really old WiFi cards and dont support or understand how Mesh systems work.  HP Printers are the worst for this issue, they never connect successfully to mesh systems without fiddling.   The culprit is usually a setting called Fast Roaming which the mesh system uses to move devices around, in short you will need to disable this feature to get these printers online, they dont like or understand roaming.

Remember as you turn off Fast Roaming, this will remove that feature for all devices.  Therefore when walking around and switching between the nodes, it may not work as well as it did before.

In the Deco App to go MORE (lower right, 4 squares) and choose Advanced, in there disable Fast Roaming and try again, with a bit of luck should now connect for you. 

Decos will put different devices on different frequencies, which are NOT considered to be the same network.   MAJOR FLAW GUYS!!!! 

This is incorrect.   Yes they are different frequencies but they are part of the same network and traffic will flow between them.   Again this is highly likely the client being older and not supporting, rather the Deco.

Is this a TP-Link software issue?

No is the simple answer.  Mesh systems require devices to support a number of different standards to work correctly and quite often devices with cheaper or older WiFi cards simply don't meet the standard, printers and IOT devices are the most common, this is a perfect example of that.