Good morning, sorry for the immense ignorance but if I have a router connected via pppoe for internet connection with configured 4 vlan, with their own dhcp servers, outgoing on a bridge what is the lan address?
Each vlan refers to its own dhcp server but the router ?
Sorry again for the stupid question 
Each VLAN will have their network address, IP pool, dhcp server and gateway. For eg I have VLAN_88, Network is 10.10.88.0/24, IP pool is 10.10.88.1 - 10.10.88.254 and gateway is 10.10.88.1 and I can access my router on that IP.
Okay
But if I have to configure OVPN ?
I connect to the router via mac because the router ip is 0.0.0
If I have to refer to router ip what ip do I put ?
How do I assign an ip to the router ?
I am confused
So if you power router for the first time you connect via MAC address and then you will see that there is default configuration applied. You can choose to keep default configuration or delete it.
If you keep default configuration then by default you can access your router with IP 192.168.88.1
If you are new to Mikrotik i can suggest you to read this topic first: https://forum.mikrotik.com/viewtopic.php?t=182373