@Fabfab74 

From what you posted, it won't use the same IP if you put the same subnet there. I remember that L2TP, PPTP and ipsec on ER605 can use the same LAN IP for the VPN IP. However, OpenVPN is a different story. 

  @John1234 

Do you have an example? 

for I do not see the mistake I may have made 

the openvpn client tells me that the connection is ok but no internet browsing or on my LAN

I tried an L2TP connection but a connection error only openvpn happens to this connected 

I tried an L2TP connection but it does not even connect: 
https://community.tp-link.com/en/business/forum/topic/539352?replyId=1056324

I tried this evening to configure a PPTP connection which surprisingly works very well on the TP-LINK

  @Fabfab74 

OpenVPN IP can use the same as the LAN IP the technical support confirmed this. If you cannot have it working, IMO, you need to check the firewall. Ping your ER605 IP when OpenVPN is on. Tracert a LAN resource. After all these, what screenshots can you provide?