@Dariuk 

Hi,

Many ISP's require a so-called "VLAN ID" to be configured on the router and without it the Internet just won't work.

Therefore, it would be helpful if you mentioned the name of the ISP and the country.

  @woozle 

Hi, the ISP name is Digi and I'm from Romania 

  @Dariuk 

If I understand this article correctly (I use Google translate)

https://www.digitalcitizen<dot>ro/configureaza-pppoe-router-tp-link/       (replace <dot> with .) 

then Digi only uses username and password for PPPoE. Therefore it should have worked the way you set it up.

Is this a brand new Internet connection (i.e. new contract with Digi) that you are trying to set up for the first time together with the Archer AX1500?

Or have you already successfully used this Digi Internet connection before?

Anyway, you could try if you can get some clue about what might be the problem be looking at the System Log of the AX1500.

1) Log into the web interface of the AX1500 using a web browser.

2) Go to the menu Advanced -> System -> System Log and then click "Clear All" to remove all existing log entries.
3) Go to Advanced -> Network -> Internet and click the "CONNECT" button to initiate the PPPoE connection again.
4) Go back to Advanced -> System -> System Log and read through the newly added messages. Normally the router should receive (RX) a PPPoE message every time after it transmitted (TX) a PPPoE message. Please verify that this is the case.

  @woozle 

I had it before on another TP-Link router

  @Dariuk 

Some ISPs have the habbit of locking their customer's broadband Internet connection to the MAC address of the first router (or network device) that was connected to it after it was activated.

So, you could try whether it makes any difference if you copy the MAC address of the WAN port of the other TP-Link router (the one that was working fine) to your new Archer AX1500.
There are two ways to do that. Either use the "Clone MAC" method (if you know how it works) or just do it manually by looking up the MAC address of the old router within its user interface and then type that address into the MAC address field of your Archer AX1500.