@Alexandre. I've tried it and no it's not working so rolled back all changes.

Couple of important parts: The deco is actively disassociating the printer (see the log in the initial problem statement).

So the AP is the side that drops the link (not the printer).

It still could be that the printer sends wrong packets though.

A Wi-Fi join goes, in order:

1. Probe / Probe-Resp
2. Authentication
3. Association
4. 4-Way WPA2 Handshake
5. DHCP
6. Group (GTK) handshake / rekeys and normal data

As written the printer sometimes appears in the DHCP list. That means steps 1–5 succeed at least occasionally (it got far enough to install keys and request/obtain an IP).

The AP then kicks it with a disassociate, so the break happens after association.

Since I don't see reason codes next to disassociate/deauth in wireless logs it's hard to tell what's going on.

One thing what I can imagine is that AP requires PMF which is not supported by older machines, but not found any deco config option for this.

  @Gabor_Somogyi 

Well, it was worth trying. Thanks for your effort and status update.

Maybe TP-Link Support will come with something, I can only offer workaround at this point.

The workaround: find standalone WiFi router, or reuse AX53 you have, or buy cheapest 2.4GHz router you could find. Attach it with Ethernet cable to the Deco unit nearest to printer. Configure that router to run in Access Point mode with SSID different from Deco mesh SSIDs. Configure printer to connect to that router SSID.

If that router is dual-band, only keep 2.4GHz enabled in it: you don't want to have unnecessary interference on 5GHz with Deco mesh.

@Alexandre. They state that this issue has been reported by someone else and they're handling it already (debugging, test software for deco, etc...).

As a workaround I've already connected the 2 routers with different subnets so kinda' hacked around.