@Pleyraki 

So you know this is the US community.  For Greece, you would want to use https://community.tp-link.com/en/ as your community.   What you may want to look at is how the static IPs are configured.  If they use MAC ID that could be the cause of this.  when connected to an Extender a device's MAC header is changed to one provided by the Extender.  This is for device compatibility.  It does affect IP address reservations that use MAC as the identifier to assigned the IP.  

Carl wrote

  @Pleyraki 

 

 What you may want to look at is how the static IPs are configured.  If they use MAC ID that could be the cause of this.

 

  when connected to an Extender a device's MAC header is changed to one provided by the Extender.  This is for device compatibility.  It does affect IP address reservations that use MAC as the identifier to assigned the IP.  

  @Carl :

thanks for the reply. 

there is no Mac Id involvement. They just take an static ip not in the dhcp pool of the main router. 
Also, you say "when connected to an extender". The problem occurs on pcs wired, not wireless. They don't use the extender at all. 
 

  @Pleyraki 

That Doesn't make much sense then.  There would be no way an extender could affect the true static IP assignment of clients connecting directly to the router with IP assignments physically set on the client that are outside the DHCP range.   IPs assigned to devices through the extender would use IPs inside the DHCP range set on the Router.  The 850RE, as far as I know the TL-WA850RE does not have Smart DHCP so it cannot create its own DHCP which is the only way i could see the extender possibly messing with your Static IP assignments. 

Carl wrote

  @Pleyraki 

 

That Doesn't make much sense then.  There would be no way an extender could affect the true static IP assignment of clients connecting directly to the router with IP assignments physically set on the client that are outside the DHCP range.   IPs assigned to devices through the extender would use IPs inside the DHCP range set on the Router.  The 850RE, as far as I know the TL-WA850RE does not have Smart DHCP so it cannot create its own DHCP which is the only way i could see the extender possibly messing with your Static IP assignments. 

maybe it's a firmware error, then. Maybe it shows dhcp disabled but it is enabled, internally. 
but, even then, a static ip pc, should not have a problem, right?  @Carl 

  @Pleyraki 

It could.  If the DHCP range (if enabled) is giving IPs outside the IP range the router provides but contains the static IP placed on the PCs then it could be what is called an IP conflict.  it could be that the Extender is trying to assign the IP address dynamically to a device connected to it and then at the same time a PC is trying to use it to access the internet as well.  A solution could be using DHCP address reservation on your Router.  This is what I call Static-Lite.  You set your computer to obtain automatically, then in the router you use the computer's MAC Address and link the computer to a specific IP in the DHCP range.  This works like a static IP in that the computer gets the same address everytime but like a dynamic IP in which you don't have to configure settings on the client device. 

Thanks @Carl , for your reply.

I see what you are saying, but the dhcp server is off on the extender. That's why I say maybe it's a firmware error.

Anyway, maybe we should drop this... thanks for your answers though.

Antonis

I finally found a solution, though it is not optimal:

I disabled DHCP server *on my router* and enabled it on the extender.

So now, all the static ip clients don't have any problems connecting, and the dynamic ip clients get addresses from dhcp on the extender.

Maybe this should be reported as a firmware error:

Although it was disabled, the dhcp server on the extender somehow prohibited connections on the router (!) for the static ip PCs.

Hope this helps!