Why do I have data connection problems after Marshmallow update?

[AC Question]

I have Lenovo A7000 dual sim phone. The first slot supports 4G, the second one shouldn't, I can activate data even on the second sim, though. The problem is, that when I want to use 4G data from the first sim suddenly I lose the signal on the sim. However, if I want to use data from the second sim, which is not 4G, I have no problems, I even have signal on sim 1 ... How is this possible? I have tried everything: switching off/on the data conn, airplane mode, phone, switching the simcards, I even did hard reset it. Nothing helped.