I have just returned a 3 pack of M5s that I spent some time trying to get working in Access Point mode - even in router mode I found issues.

Some of these "issues" may not be a problem for you but here is a list of what I found to be issues in my environment where I wanted to set up wifi behind a router and firewall.

I had read what I could find before buying but did not see information about any of this as the documentation is so minimal.
1 The app requires access to everything on your android device when installed. Why?
2 The app is unreliable.  Sometimes it said it could not see the deco system even though it was connected to the SSID from the main unit.
3 Allows devices to use WPA to connect to the wifi whether you want to or not.
4 Unable to give the deco a static IP address in access point mode.
5 The performance and range of the deco system is poor in my opinion.  I am getting faster speed and greater throughput using an extender! 

6  Devices, phone, tablet or laptop will not readily switch to the deco unit with the strongest signal. Tried several devices and even a 20db difference was not enough. I am aware that sometimes this is device dependent vbut i tried numerous devices with the same result.
7 If your internet connection is down (or the tp link site) you cannot make any changes to your environment.
8 Traffic goes from the deco units to google analytics. Why? What? Traffic also goes to tp link sites on a non-standard port.
9 The guest network still transmits in access point mode even though you cannot connect to it.
10 I was aware that some of the functionality would not work in AP mode but the "etc" comment in the description needs to be changed.   There is virtually no options available in access point mode.

It may be that the system will work fine for you if you just plug it in a basic mode in a simple straightforward environment....anything beyond that think carefully before getting the M5.