The situation is very strange, I am aware of that. Let me summarize again the behavior:
There is no internet access on any device connected to RB951-2n. In LOG there are constant connecting and terminating entries. This is not a new installation, with this configuration router is running for more than a year (only routeros version was regularly updated). Curent RouterOS version used is 6.33.3. I tested the gateway on ether1 and ether5 and the situation is the same.
I called my iSP provider and they told me that there are no restriction in using a router but they can't offer me support for router configuration, I must connect the external cable (provided by iSP) directly to a PC and if there is no internet access then a ticket will be opened considering that there is a problem from their side.
When i connect the external cable (provided by iSP) directly to a PC and use DialUp PPPoE (in windows 10) the PC connects and internet works fine. So... the conclusion is that cable and service are ok.
I wold like to say once more that this situation occurred Friday evening without any intervention to the setup. I just noticed that internet access is not working and checked the LOG. The router is using this setup for more than a year.... Normally my first suspicion was for the iSP but because Win10 DialUP PPPoE is able to connect the problem i think is back to router side (or somewhere in between ?!).