I am using my ISP provided router as the gateway device for my home network, with 3 Deco M5s connected in "Access Point" operation mode. Having the Deco units in "Router Mode" is not practical for me as it segregates the network and means I have to apply policies to both my ISP provided router and the Deco units. My desktop is connected to the ISP provided router, and other devices connected through the Deco units. When trying to communicate with other devices on my network (SSH, SFTP, printing/scanning, etc.) from my desktop, I frequently experience dropped packets which breaks the connection. The only way I have been able to mitigate this is by reducing the MTU size on the devices I am communicating with which is not an ideal solution. So it seems that large packets are being dropped rather than fragmented when over the MTU size, presumably after having the additional Ethernet headers appended. However, I'm not sure where in the network, nor why, the packets are being dropped. I also don't know how to go about finding where the issue lies.

Is it the fault of my ISP router, or the Deco, or neither (expected behaviour)? How would I find out?

Here's my network topology (other host devices have been omitted):