I have a similar issue. After I bought and powered it on the first time the unit picked up GPS signal with no issue. I turned the unit off and went fishing the next day and the unit never got signal the whole 4.5 hours I was out. I thought it was maybe because I never “activated” the Navionics chip. So I did that and the unit still never received GPS signal. It just showed a grey arrow on the map.
after seeing your post I am going to try and return it because I feel there must be an issue with the MX-7 GPS