WiFi is faster with Bluetooth turned off due to interference. Read this post and the following 4 posts:
https://xdaforums.com/t/ac8259-cpu-based-hu-problems-questions-mainly-yt5760.4171219/post-89643363
This video shows cutting the antenna slightly improves signal. I guess to better match wavelength. No idea if it works but comments seem to show some success:
https://www.facebook.com/BinizeOfficial/videos/400961157941834/
Someone on 4PDA also reports faster WiFi with longer antenna:
Head units on MTK AC8257 processor, MCU: HDBEC/TS-G5 - 4PDA
HU includes 2 cellular SIM card slots: One internal (SIM1 require disassembly to get to it) and one via external cable (SIM2). Some people received units with China Unicomm SIM card installed in SIM1 and common solution outside China is to disable SIM1. Mine came without SIM card in SIM1.
My HU cellular can only connect T-Mobile EDGE (2G) in US but appears to have no data access (or very slow). IMEI check on US T-Mobile site shows unsupported ZTE phone. AT&T SIM card shows 3G although I haven't tried a working SIM card to check live data connection. 4G/LTE works at least in central Europe.
Here is a review of all the LTE bands supported by this HU and regional compatibility:
https://xdaforums.com/t/ac8259-cpu-based-hu-problems-questions-mainly-yt5760.4171219/post-86960289
Android "Settings" -> "Network & Internet" -> "Mobile network" -> "Enhanced 4G LTE Mode"
SIM must be inserted and HU rebooted afterwards. Provider, your tariff plan and your mobile network needs to support VoLTE of course. VoLTE needs strong LTE signal.
It seems not all HUs offer this option, only a 8GB RAM with 64bit OS has been verified so far, 2/4GB HUs with 32bit OS doesn't seem to have this option (see here and here).
Source: https://xdaforums.com/t/ac8259-cpu-based-hu-problems-questions-mainly-yt5760.4171219/post-89675218 and following posts
If not working, try reset as mentioned here:
https://xdaforums.com/t/ac8259-cpu-based-hu-problems-questions-mainly-yt5760.4171219/post-89764470
A few people have found poor SIM chip card soldering. Here is a pic. Best to get someone who knows Chinese to confirm the colors above the solder joints.
https://xdaforums.com/t/ac8259-cpu-based-hu-problems-questions-mainly-yt5760.4171219/post-86692371
Some connection difficulties exist. Find out what PIN the Bluetooth ELM adapter uses and change the head unit's default Bluetooth password to match it. After that, you can change the password back to your preference and OBD2 will stay connected.
After it is connected, Android won't show the connection, but Bluetooth app will show connected and OBD2 app (e.g. Torque Lite) will connect.
Source: This post and reply posts #992 and #993: https://xdaforums.com/t/ac8259-cpu-based-hu-problems-questions-mainly-yt5760.4171219/post-87371089
ZLINK license is usually purchased with original HU. If you do not have ZLINK license, you may not be able to activate it. Don't know if contacting ZLINK app developer (zjinnova) can be successful.
I believe ZLINK has 2 activation methods. Offline and over the air. There is a license code in the HU and ZLINK can activate without network connection. When network is available, ZLINK can activate over the network.
If you format the flash memory, the offline activation code will be gone and require network to activate.
When my HU's data got wiped (from prior firmware version installation and/or random firmware crashes), ZLINK activation seems to automatically recover (link).
If general, if you have activation error after firmware updates, just turn on network, reboot HU and relaunch link a few times.
You can also note the ID/SN/Key/Key-Code/Build-In-Code inside the app (e.g. at "Setting" -> "Device Info" or "About".
Some people complain ZLINK5 doesn't have do full screen (override the android bar on top). 3.9.48 and 3.9.27 does do full screen per this post. Links to both APKs below:
https://xdaforums.com/t/ac8259-cpu-based-hu-problems-questions-mainly-yt5760.4171219/post-88662171
1.80.1 renamed ZLINK to X-Car and does have full screen (link). However, 1.80.1 changed system partition and require PC Flashing. See also "Archived versions" below.
ZLINK 3.9.27 is archived here:
https://xdaforums.com/t/ac8259-cpu-based-hu-problems-questions-mainly-yt5760.4171219/post-87638245
ZLINK 5.3.31 is archived here:
https://xdaforums.com/t/ac8259-cpu-based-hu-problems-questions-mainly-yt5760.4171219/post-89710657
X-Car 5.4.62 is posted here:
https://xdaforums.com/t/ac8259-cpu-based-hu-problems-questions-mainly-yt5760.4171219/post-89700454
For 1.80.1+ users, 9/2/2024 firmware update switched to X-Car as default.
For 1.73.8 to 1.75.1 users, 5/3/2023 firmware update switched to ZINK5 as default.
I'm not ZLINK user (I have activated license but do not use it), but it seems that alt least older versions (end of 2022) had problems:
https://xdaforums.com/t/ac8259-cpu-based-hu-problems-questions-mainly-yt5760.4171219/post-87688013
@hyz_123 tried various versions of ZLINK and posted much info. Can see all his posted contents via the profile link.
In older firmware ZLINK call volumes is very low (explanation link). More recent system firmware seem to have this fixed.