I had this problem with a Roku 4 Ultra Media Player.
Tried all sorts of things to resolve this issue.
My Ultra Box I bought on ebay. A US Version. Connected it all up to 4K tv and started the activation process.
It set language and network options, then went to update software. Its downloads 100% then says restarting. Within a min or so, we are back to the language network settings and then the update.
I tried all sorts of things listed. HDMI cable, different network, mobile network, different tv ect.
I then bought a new power adapter, and hey presto. Straight through the update and activation. Now working fully updated fine.
Anyways, just thought I would post this, as this was my solution to this issue and it might help someone else. the adapter cost all of £9.95 so all good.
Hope this helps