...most ISP connections I know of, that use PPPOE require to use a seperate VLAN (i.e. German Telekom, VLAN-ID = 7)
But not all. Log entries (connecting ... authenticated, connected, terminating ... disconnected) actually show that VLAN is (probably) not a problem since PPPoE client can talk to ISP's PPPoE server. Some other problem is more likely. OP should increase verbosity of pppoe logging, perhaps something useful will pop out.
how can I achieve that logging? sorry just starting Mikrotik tho
You can add the pppoe debug logging topic using
/system logging add topics=pppoe,debug
. (Or when using winbox: Go to the system>logging menu. Rules tab. Click "+" and add the topics pppoe and debug with action memory).
With regards to your issue with sending the traffic over the second link:
First of all you have to use the pppoe interface as your out-interface in your src-nat rule, not the WAN2(ether2) address-list. Also add your pppoe interface to the WAN2 address list (presumably for firewall reasons). Second of all is that you now have two equal default routes, and only one will be active. One through ether1 and one through the pppoe interface. The selection of which route (and therefore interface out to the internet) is active is thus either ether1 or the pppoe interface. I see that you have tried to fix that using the src. address list in the src-nat, however src nat happens AFTER the route/interface selection. What happens now is that a packet from the 178_network might decided to go out of ether1 (if that route is active) and it will not get src-natted (private ip leak. probably blocked by your isp). Or it might decide to go out of the pppoe interface (if that one is active), in which case it will be correctly src-natted and work correctly (if you change the src-rule to use the pppoe interface and add the interface to the list). If the pppoe route is active, all traffic from the 88_network will fail, if the ether1 route is active the 178_network traffic will fail.
To fix this you need to remove the src address list parts from your src nat (thats only making it worse for now) and configure policy routing. See https://wiki.mikrotik.com/wiki/PCC
. Basically you have to mark traffic coming from, for example the 88_network, with a routing mark for WAN1 and for the 178_network for WAN2 using mangle rules in the output and prerouting chain. This is hard to do with PPPoE though since you cannot create a marked route dynamically from pppoe if I recall correctly. You might need some scripting in the ppp profile for that or check out viewtopic.php?p=814682#p814682