@woozle I'll Ty to re-install the firmware then. 
The router model is ZTE ZXHN F679L

  @CM17X 

Re-installing the same firmware won't help. You would need a newer firmware that fixes the problem.

But here is something you can do.

This video https://www.youtube.com/watch?v=KvkZa0Z6mg8&t=842s shows the admin interface of a similar model ZTE ZXHN F670L and at timestamp 14:03 a "Mesh Enable" option is visible.

Can you please check whether that same menu containing the "Mesh Enable" option is available on your F679L?

In case it is, then switch it off, click [Submit] and then setup your RE220 again. That should solve the problems.

  @woozle I'll try that option, but need to wait for the ISP to respond. It's locked. I've tried the default user/password with no avail. Althought the RE200 itself says my router doesn't work with mesh, it would be strange that the option is there and active or something.

  @CM17X 

I noticed that TP-Link has released a firmware version 1.0.50 for the RE220 V3.

https://www.tp-link.com/en/support/download/re220/v3/#Firmware

Although the release notes don't mention anything in this regard, perhaps they did fix the issue that EasyMesh is still active even when the EasyMesh switch is in the "off" position.