Cant activate a connection to mobile device while screen off and tethering

I have older mobile phone in the car with a teamviewer host that I connect to, unattented method, very well when on WIFI from the house in the garage using PC or my current phone.

However if I try to connect to it outside of home while thetering off my mobile data connection while screen is off, no connection happens. If the screen is on on the old car mobile , connection happens fine. Essentially it seems the echo service can not activate connection while on a mobile thetered connection. Any idea whats missing? Thanks.

Both phones are samsung, with host add-on for samsung applied.