If you is using one adapter that is using GIPO as serial communication you need disabling the local console logging for freeing the hardware port for your adapter (or its gets all thing wath the system is logging in "the mouth".

Freeing the GIPO comport port from local console and loging

Open one consolle local or with SHH.
Execute: sudo raspi-config
On PI Zero the sequence is:

5 Interfacing Options  Configure connections to peripherals

P6 Serial      Enable/Disable shell and kernel messages on the serial connection

Would you like a login shell to be accessible over serial?
<No>

Would you like the serial port hardware to be enabled?
<Yes>

The serial login shell is disabled
The serial interface is enabled
<Ok>

Save and reboot your PI Zero

On PI3B+:

3 Interface Options    Configure connections to peripherals

P6 Serial Port Enable/disable shell messages on the serial connection

Would you like a login shell to be accessible over serial?
<No> 

Would you like the serial port hardware to be enabled?
<Yes>

The serial login shell is disabled
The serial interface is enabled
<Ok>

Save and reboot your PI 3B+

Now we have one software serial port that is working . . . .
NOT so good.

Enabling hardware UART to GIPO comport port.

The hardware UART port is default being used for BT and the GIPO is using software UART port but can being changed or disabling the BT port.
Open one consolle local or with SHH.
Execute: sudo nano /boot/config.txt
In the end of the file you have the last comport changed.

Make sure this is in one of the pi3-miniuart-bt or pi3-disable-bt and uncommented (but not both) and enable_uart=1.

enable_uart=1

#Use software UART for BT
dtoverlay=pi3-miniuart-bt

# Disabling UART for BT
#dtoverlay=pi3-disable-bt

Save and reboot your PI.

Now we have HW UART to our adapter and SW / Disable UART to our BT but its OK then its not the primary use of our PI with ZHA.

ModemManager

ModemManager is not default installed on PI but can being installed of the user or ad dependency of other applications being installed by the user. On NUC and other hardware platforms its normally installed and need being removed for not hijacking our comports on the fly.
Open one console local or with SHH.
Execute: sudo apt-get autoremove --purge modemmanager
Save and reboot and the ModemManager is away.

Using WiFi on our IP3B+

I have my PI3B+ on the side and it dont have Ethernet connection there so i using my normal WiFi.
If using the same WiFi network name for both 2.4 Ghz and 5 Ghz its oft jumping between them and interrupting the network traffic plus the 2.4 Ghz can making the Zigbee degrade.

So i have doing one extra virtual 5 Ghz WiFi network with the same network settings as the "normal" with the name normalwifiname plus G5 in the end (normalwifinameG5) and having adding it as WiFi network and deleting the "normal" (normalwifiname) 2.4 Ghz WiFi in my PI3B+.

This way it can only using the 5 Ghz and not jumping between 2,4 and 5 Ghz and not interference with my Zigbee network on my PI3B+ and all other WiFi devices can using both 2.4 and 5 Ghz network with the same name and jumping around as they like.