I currently have 3 Deco M4 units, connected to my default ISP router as an Access Point. 

Originally, bought these as a router replacement, hence the original setup was:

ISP Modem - Ethernet - Main M4

- Air - M4 2nd Unit - LAN - Printer

- Air - M4 3rd Unit - LAN - Thermal Receipt Printer

Everything worked well for a couple days, and suddenly the main deco turned Red, unsure what led to this and any restarting of the units did not work. Was using this in a small office so desperately had to swap back to the original ISP router which had some blind spots and had to reset the Deco M4s into Access Point Mode. So now the setup changed to 

ISP Router - Ethernet - Main M4

- Air - M4 2nd Unit - LAN - Printer

- Air - M4 3rd Unit - LAN - Thermal Receipt Printer

After changing to the Access Point mode, the network was working again, but comes with a speed cap since the QoS bandwidth setting is no longer accessible and editable for Access Point Mode. my original ISP speed is 500 Mbps, but now maximum outside we can only get 50-70Mbps, and occasionally even 10mbps and below.

An issue I faced with this setup was that sometimes the app itself tells me "Unable to connect to the Internet", and the Decos are red as well. But if I configure the devices DNS to manual they can still access the internet weirdly. I'm not too tech savvy in networking so I was really hoping someone can help me with this as I'm really lost as to how to change this but the insecurity from the last round of mishap from the internet when using router mode for the M4s is holding me back, as a slow but working internet is better than no internet at all. The ISP router has been tested and we can definitely get 500mbps if we're in the router room. But once we step out of the room all connection gets slower.

The office internet and user specs are:
Around 50 devices

2320 Square Meter Space

500Mbps Internet