Oversea fund for evidence of savings

I was under Tier 2 visa until April 2018 when I switched to ILR (after 5 years on Tier 2). My wife and son joined me in the UK (July 2017). Their Tier 2 visa will expire in 2 weeks time. During their first application, I used maintenance letter from the company so I did not provide any evidence of saving. I thought the situation is the same for this time to extend their visa. Without realising it, the situation is different because I am with ILR status now, so the company cannot provide any maintenance letter. The issue is I dont keep enough money in my UK bank account for the 90 days period. I have oversea investment (unit trust) which certainly satisfy the amount UKVI required for the 90 days period. This is stored in the financial institution oversea. Can I provide the statement/ letter from them showing that I have the required fund for the 90 days period. Is this acceptable? Please advise. I dont want to go with FM route which seems to be more difficult with more documentations.