Hiya @Alicechet & Welcome To The Cuckoo Community !!!
If theres a solution at hand then we will find it together thanks go to @RyanK92 for stepping in so far.
Please Dont Worry Too Much about your phone not being compatible with the Cuckoo Router as I highly doubt that is the case,I have Access to 2 Motorola Smartphones in my household that I can test if I need to.
I Doubt it will be the phone at fault as I also have had several other models of routers working with older & newer devices even in 2021.
Now I suspect the reason your device is not connecting is because the cuckoo router is setup to use the 5GHZ Highspeed Frequency by Default which should in theory drop down back onto the 2.4ghz frequency if it cant deliver the speeds required to your device.
So to test this theory can you open a web browser on your phone while connected to the wifi thats failing or from a working device connected to the Cuckoo Router and do the following.
Type in the following web address 192.168.1.1 and hit Go or Enter etc.
This will then connect you to the Technicolor Routers Home Page
At this point you need to sign in using the “Access Key” on the Bottom of The Cuckoo Router as a password.
When you’ve logged in correctly it will say “Admin” in the Top Right.
From here I need you to tap or click the “Wireless” Tile on the end of the top menu tiles which will either display a Settings “Cog” or take you straight in to its options.(it will also state your Wireless Name and say Auto/Shared 5Ghz)
On the left hand side menu it will show you the name of the Wireless Name you may have set when you signed up and there will be two of them. The first one will say 2.4ghz and the Second will be 5Ghz on a header.
Go into the second wireless name which will be the 5Ghz menu and on the right side menu you should see a button that shows that its turned on & Green.
What I need you to do is “Turn This Off” by tapping/clicking it and it will go Grey when you do this correctly.
A Green “Save Changes” Button will appear at the bottom of the menu to make sure this setting sticks please tap/click this.
Now when you exit out of this screen you should see the Wireless Menu Tile in the Top Right Has Seperated into Two Seperate Names one will be Green and the Other will be Grey.
Now this is where it gets Fun
If you now scan for a wifi point again on your device you should see a new version of the Wifi Name that you were connected on previously but your not connected to this one at the moment.
Tap this name and input your Wireless Password that you chose or look on the sticker on the bottom of the Cuckoo Router for reference and remember it maybe case sensitive.
Hopefully this will be the 2.4ghz Band your connecting too
If you have done this correctly it should fire up and start giving you Internet Access within a couple of minutes…I normally hear the sounds of many messenger dings and bleeps and plops at this point.
You will also notice that Only One Wireless light will be lit up & flashing on the Router at this point…Don’t Worry this is Fine as the second Light/WIFI Band is turned off on the previous steps.
The only thing I must also point out is that the other devices you may have had connected on the previous 5GHZ wireless name will need to reconnect in the same way above as they will be disconnected and will need to reconnect to the new wireless name.
Let us know if your Motorola G4 Play is still playing up after completing the Above steps or not.