What are the network troubleshooting algorithms?

Question

Computer networks

The network uses different technologies such as ADSL, GEPON, Ethernet, wireless antennas.
ADSL works over PPPoE. and also there are stupid networks, which are also via PPPoE.
In GEPON, Ethernet dynamically receive ip, using QinQ technology.
What problems can users have with access, speed or something else and how can they usually be solved, perhaps there are some troubleshooting algorithms, what to check first and what to exclude?

Answer 1

Check the elements of the OSI model from the bottom up.

Answer 2

And who are you - an operator or a large distributed corporate client? If it's the former, you'd better build relationships within your team and learn from more experienced colleagues. In any case, in most cases, problems on Ethernet are looked for on switches, on G(E)PON in the control system, on ADSL - on DSLAM. Less often on BRAS or in billing.
And yes, QinQ IPs are not issued.