LAUNCHXL-CC26X2R1: Zigbee 3 router to Zigbee HA coordinator

It should be backward compatible by default.

I cannot connect them. The coordinator is a CC2531 USB dongle and the router is a launchxl-cc26x2r1. I know that the coordinator has connected with other devices that had zigbee 3, but I cannot understand why it doesn't connect with the one that I have right now

I would suggest you to use sniffer to check what exactly happens over the air.

I set requestNewTrustCenterLinkKey to FALSE but it still won't connect. I also changed the channels because they weren't the same

Make sure to factory reset all devices and have the same channel configured, then provide a sniffer log of the failed commissioning process.  Make sure that the HA coordinator has permit join enabled when attempting to commission nodes into the network.

Regards,
Ryan

Can you specify which Z-Stack do you use on CC2531 and LAUNCHXL-CC26x2R1? Can you also provide sniffer log to check this issue?

I know that other Zigbee 3 devices have connected to the HA coordinator so it can't be it's fault. I have tried after factory reset without any results.

I am using the latest sdk provided for the launchxl-cc26x2r1 and ZSTACK-HOME for the cc2531.

At the moment I can't provide a sniffer log. I have opened another thread to fix the problem that I am having with the sniffer. When I get it up and running I will send the sniffer log

Just curious, why are you evaluating the deprecated Z-STACK-HOME solution?  Zigbee 3.0 networks are capable of allowing HA devices to join their network.  It's better to upgrade the ZC/ZNP than downgrade the joining devices.

What is the channel on which the HA network is formed, and what Primary Channels are selected in Z-Stack for your CC26X2R1 device?  Have you tried debugging the SimpleLink SDK project for further clues as to why the joining process is failing?  Are you able to gather any information from the CUI through a UART terminal connection or MT interface?

Regards,
Ryan

Basically the company that I started my internship a few months ago still use Z-STACK-HOME and the task that I have is to is to make two zigbee 3 devices communicate through a custom app that they have created (the other thread that you are helping me: LAUNCHXL-CC26X2R1: No router response at AF commands 2 - Zigbee & Thread forum - Zigbee & Thread - TI E2E support forums). And they told me to try to connect the Z3 router to the Z-HA coordinator to see which board doesn't work right because they have used a Z3 clock on their network and it was working fine through their app

The network is formed in channel 15 and I have modified my Z3 device to function in that channel. I have tried debugging with no results. When I implement MT the CUI doesn't work when I try to access it through PuTTY 

You should start by importing a default ZR example (i.e. not MT or CUI altercations), change requestNewTrustCenterLinkKey and Primary Channels, factory reset all devices, enable permit join/open the network, and commission the ZR.  A sniffer log will help determine what the issue is.

Regards,
Ryan