Ha this reminds me a period years back, traveling to Egypt from UK (kinda after they had plane "troubles" over there). Basically every airline demanded you put every electronics (so laptops, cameras etc ) in hold, as opposed to carry on. Of course they also absolutely refused to take any responsibility for it. Insurers (including that one UK company that insures UW gear) also refused. Was a real limbo then xD
But in terms of your problem - I cannot speak for the insurer, as that one will indeed depend on the contract signed, however airline cannot refuse compensation using "valuable items" excuse and is liable for every single item in your luggage, albeit only to a certain total sum. It is usually 1288 SDR (about 1600 EUR / 2300 CAD) as per the Montreal Convention, however US airlines are also governed by DOT and the limit is much higher, 3800 US (at least for domestic routes, I am not entirely sure if that limit will also apply to international or the US equivalent of 1288 SDR instead).
My assumption would be that this would classify as a "lost baggage", hence the below would apply.
(From an absolutely excellent page, very well covering NA (CA+US) and EU. I found it very helpful when dealing with a certain airline from that region, after it has lost my whole bag with all my dive gear inside (luckily found at the end but only because I flew 3 countries away to look at the airport myself, with no help from said airline). There is lots of practical guides, sample legal letters, as well as instructions how to file cases with small claims courts, sadly often the only resort against scummy airlines.)
https://airpassengerrights.ca/en/practical-guides/baggage/lost
And in regards to United trying to use the good old "valuables shouldn't be in the checked lugagge bla bla bla" excuse (note the DOT advisory link as well!):
https://airpassengerrights.ca/en/practical-guides/baggage/lost/faq
You can also find bits on wikihow:
https://www.wikihow.legal/Take-Action-for-Stolen-Airline-Luggage
Good luck and be persistent, that is the only thing that will get you anywhere. Airlines are not interested in helping you, for them getting rid of the customer equals resolving the issue, and that is often easiest done by either saying no, regardless of the law, or not even bothering to respond. Doing something actually helpful is seen as too bothersome to those poor little airlines, claiming tens of billions in government help every time a crisis shows up.