By my experience Jolla can suffer from bad network connection while not bad enough as to keep you in doubt for a while.

Luckily mine has shown deterioration after repair by Jolla, so I immediately figured out that during the process something must have gone wrong with the antenna connection.

The problem was that everything worked but not exactly as it should. I changed SIM cards to another provider. Then I compared 2 phones next to each other and found the disadvantage for the Jolla.

With this knowledge I could send it in for warranty repair and things have improved back to normal level.