Mine was the same, all hdmi ports, random signal lost, it got to the point where I was going to be taking the TV back for a refund, it was totally spoiling my viewing.. A real pain, the firmware they can email to you, put it on a USB stick, I think I used fat32 for the format, but you might find in NTFS format works too, the update was a very recent update file, 100 percent no issues since, I've used my nvidia shield tv set to 4k output to watch various movies, lots of Netflix, not once as the no signal issue appeared, previously it was frequent.
If you don't have a USB stick, they will offer to send you one in the post for free.
I'd say the Hisense customer service over the phone was great... Give them a call update the TV... I found I had to re enter all my app passwords, I think WiFi was still there, so be ready to re-setup your TV.
If you have your colours set, make notes of all your settings, as they will all default back to the original settings.
I took photos of mine, made it quick and easy to put it all back to how I liked it.
Freeview requires you to set the county setting on the TV to UK, if not, freeview will not find any feeds.
Don't forget to set the HDMI ports to advanced or whatever they call it, for it to work correctly with 4k, as it will default to standard mode after you update the firmware.