@Kevin_Z thanks for the reply.

Yes the first time I tried to use it as an access point all the configurations of the DHcp were on AUTO, then, as it was not working, I searched and found the above mentioned thread and tried disabling it. Both configurations fail to assign ips, maybe after some time

LEDS where all properly on, including the internet one, and from the tether app i could see that the router thought Internet was working, but my phone was getting no acess to it.

On the ZTE modem i could not find any settings, even on the advanced ones, that were blocking the AX55, I even tried resetting it to factory settings to no avail. I have also tried assigning the same static ip (192.168.1.4) from the modem, just to be sure. I am pretty sure that there weren't any setttings to block the router because usually some devices were working and connected to the internet (like for example the laptop I used to set it to AP mode), the main problem was connecting new ones, or, for example, my phone was working right after I configured the router, then i went away for 1 hour and it would not be able to connect again (could not configure ip error from android settings).

In router mode everything was working fine, even for long periods after configuration, but I needed an AP to avoid subnets.

Another thing i did not mention is that i also tried to connect the ethernet cable to a LAN port instead of a WAN port, since i read a post on reddit that suggested it.

Anyways, today i pulled the trigger on a new Mercusys mesh wifi system, and it is working perfectly, but to be fair it is not in AP mode, i just dont need it to be since the mesh makes one whole subnet for everything. I am planning to return this AX55 but if anybody can think of a solution i could try it for the sake of finding out the problem

  @Kevin_Z 

Yes you are right, the ZTE should support easymesh, but I am not sure if it is actually tp-link's Easymesh. I did try to pair the AX55 as a satellite but i did not have any success in getting them to talk. (I actually chose this model because I thought it would work with my ZTE modem). If I recall correctly the procedure I did was:
Reset the AX55
Reboot the modem
press WPS on the modem
Press WPS on the router

I tried many times (following tp link configuration video) but no success. The only settings in the ZTE modem regarding easymesh are the following: 

so i don't think there is anything i can do from the modem. What i suspect is that ZTE's Easy Mesh is a different protocol from TP-LINK's EasyMesh. Also connecting to the AX55 after the reset and setting it to satellite mode does not solve the problem.

Now I'll try to connect the AX55 to the modem with the setting of easymesh disabled and see if it works in the access point mode, but I'd rather not reset the mercusys system right now since i have all the ip's configured correctly and i don't wanna reset it at the moment.

  @Kevin_Z i have tried that and it seems to have fixed it! I havent connected as many devices as before, but the ones that i connected over the last two days. had no problems. Still was not able to make it work in mesh mode but the original problem of this thread seems to have been fixed.

  @Kevin_Z 

Yes it is, but it is acting as the router, so i guess that could also be the reason.

I'm still wondering if the ZTE modem (ZXHN F6745M) is supposed to be able to connect to the ax55 or not, because i liked the VPN implementation the AX55 has, which the mercusys does not have.