[CM12.1][5.1.1][D618/D620] Unstable wi-fi

Hi,

I installed CM12.1 rom (from Stock 5.02) following the instructions from the OP. Installed KK bootstack before the ROM.

But the data network is really unstable: it takes a long time to connect to data, 4G works rarely. I tried several options for "preferred network type", but no one works well.

Do you have any suggestion?
Thanks