Please help, my phone stuck on 2G

Try this path:
Settings - (More) - Mobile Network - Prefered Network Type.

I have a Dual SIM phone.
Under "Settings - Dual SIM Management", the SIM cards are listed and the currently supported standards indicated for both SIM card.
In my case, only the "Default Mobile Data" SIM card  supports 2G/3G/4G. The other currently supports only 2G. If I change the "Default Mobile Data" setting, the other SIM card supports only 2G.

The Settings path is probably different at your phone. But maybe you can find anyway those settings.

Andy Passenger wrote:

Try this path:
Settings - (More) - Mobile Network - Prefered Network Type.

...

The Settings path is probably different at your phone. But maybe you can find anyway those settings.

Thanks Andy, appreciate your help. I followed your suggestions, found the Settings path, but still no joy. Fortunately I heard free wifi is abundant throughout the city, so hopefully I can rely on the free wifi most of the time.

Check again the settings under:
> Data usage : Set "Mobile data" to ON
> Cellular network: "Data Roaming" Try to set to ON (my SIM don't need this option)
> Cellular network > Access Point Names: What is indicated?
> Cellular network > Network operators: What is indicated?
> Cellular network > Preferred network type: What is indicated?

Have you check the text messages?
Usually you must put a PIN code in order to activate the SIM card.

Have you load a data package?
Some data package reduce the speed after the data package is depleted.
Check the data balance and package validity by calling *101#.
Check the promotion data balance and package validity by calling *102#.

If your SIM card is expired it is deactivated and you need a new SIM card.
If the data package validaty is expired, you must load a new data package.
The SIM card will not expire (and deactivated) as long as you top-up money or load data packages (e.g. minimum each month).

Here is a link with more information:
http://www.cellularabroad.com/vietnamSIMop.php

If you still not have 3G/4G I recommend going to another phone shop.
When you go to a Viettel shop, they can temporary insert another SIM card for checking.
Or buy another cheap SIM for 100-200k Dong for checking.

Andy Passenger wrote:

Check again the settings under:
> Data usage : Set "Mobile data" to ON
> Cellular network: "Data Roaming" Try to set to ON (my SIM don't need this option)
> Cellular network > Access Point Names: What is indicated?
> Cellular network > Network operators: What is indicated?
> Cellular network > Preferred network type: What is indicated?

...

If you still not have 3G/4G I recommend going to another phone shop.
When you go to a Viettel shop, they can temporary insert another SIM card for checking.
Or buy another cheap SIM for 100-200k Dong for checking.

Hello Andy, my settings are as follow

Data usage : "Mobile data" = ON
Cellular network: "Data Roaming" = ON
Cellular network > Access Point Names = Viettel-wap-gprs 1
Cellular network > Network operators = Viettel
Cellular network > Preferred network type = LTE On (Automatic LTE/4G/3G/2G

Text message is working and data package is loaded. All settings seem correct, everything works, it's that the internet is really slow.

I will try another phone shop. Thanks very much Andy.

Can you change the Access Point Names from Viettel-wap-gprs 1 to v-internet?

I think the ZTE ZMax Pro supports only the UMTS band B2 (1900 PCS) and not the UMTS band B1 (2100).
Maybe this is the reason that the default Viettel APN not works.
It's just an idea. Not sure about that.