How to check what is wrong with my mobile connection? I have a Samsung J3 and recently I notice some irregular 'blackouts' in the mobile connection. Suddenly, the connection is completely interrupted, then the connection is really slow (like 1.9 Mbps in the center of Geneva!), and then it works just fine (with download speeds  well above 10 Mbps). 

I restarted the phone, select 'Select automatically' in the available network settings, and talked to the provider. They admitted there was a network problem, but I am using it at two very different places. 

So how to find out if it is my phone or the network?
Tracing networking problems in mobile networks is pretty complicated. 

Therefore from your perspective there is only one way to find out if it is your phone or not: Use a different phone (different model preferred) using your SIM card at the same places. 

If you encounter the same performance drops you know that it is not your phone. If not it may be your phone but that is not for sure.