Thank you @Ivaylo to suggest reseting the router to the factory defaults. I have already done it but it didn't solve the problem. (see screenshot)

My hope was to go back to the previous firmware 1.4.7 that was connecting properly but trying to local upgrade with the file to c80v1-up-noboot_2020-09-07_17.37.00.bin returns the message : "Failed to upgrade the firmware. Please check the configuration file and try again."
 

Hi @Kevin_Z,
the internet is actually working : clients (wired or not) connect to the internet through the Archer C80 and have internet access without restriction. Only the network map page (http://192.168.0.x/#networkMap) displays the problem with the red "x" symbol and the Archer C80  System Log shows the error : "ERROR 0days, 00:00:22, [mcastproxy]Lan dev not find."

Even by a factory reset, I couldn't get rid of the "no internet connection" status.

I did try to upload any of the three UE firmware available on the page  https://www.tp-link.com/de/support/download/archer-c80/#Firmware  and got always the same error : "Failed to upgrade the firmware". 

The two functionalities that I'm missing since I have the release 1.5.0 are :
- The list of the connected clients on the network map page (http://192.168.0.x/#networkMap) shows only the clients connected to the access point and no more all 192.168.0.x clients of the LAN. Same in the Tether app.   

- The firmware upgrade is blocked. Either an online upgrade, nor an file downgrade is possible. See screenshot. 

@Kevin_Z 
C80 in AP Mode.
ISP = voo.be

WAN connection type is not PPPoE and DDNS is disabled. I have a Public Ip

@Kevin_Z 
Steven helped me and found the solution to get internet connected : Status and online firmware upgrade are back.

Solution is to connect network cable on blue connector instead of yellow port.
Thxs to Steven

##########################################################################
#                    Archer C80  System Log 
# Time = 2020-12-01 13:18:11 11219s
# H-Ver = Archer C80 1.0 : S-Ver = 1.5.0 Build 201022 Rel.53112n(4555)
# L = 192.168.0.11 : M = 255.255.255.0
# WAN = DHCP : W = 192.168.0.11 : M = 255.255.255.0 : G = 192.168.0.1
# Cnt = 10624, Free = 10592, Busy = 32
##########################################################################
  0days, 00:00:03, [lan]LAN: Attach mirror0 to stack.
  0days, 00:00:04, [lan]LAN: Set interface mirror0 ip=192.168.0.11 netmask 255.255.255.0.
  0days, 00:00:20, [httpd]Http server start!
  0days, 00:00:22, [lan]can't find arp entry
  0days, 00:00:22, [mcastproxy]Lan dev not find

arp messages are still not understood so that only clients of access point are listed. This affect only the status not the functionnalities. Hope to get this back in next firmware.