Cell phone does not connect to cellular network after disconnecting from WIFI

When my cell is connected to WIFI everything works fine. When WIFI is disconnected (leave area or WIFI goes does down) cell doesn't connect to cellular network and I'm getting "Service not available" errors.  If at this moment I'm on a call - this call gets dropped.  Sometimes it connects to cellular after 3-5min, sometimes I need to restart phone to get connected to cellular. Very annoying and unreliable. Happens on Motorola and Google pixel phones so it's not HW dependent.