Unless you have a different agreement with your investors, you should stay in cash (say, government bills of less than 3 months maturity) or at most very short term government bonds matched to your liabilities (e.g. a 1 year bill if you are pretty sure you won't need the cash for one year).

The last thing you want to do is tell investors that you lost money investing the cash that they gave you. For example, US treasury inflation-protected securities maturing on 4/2023 pay CPI inflation minus 3%. Say you buy these securities thinking that it is a good idea to protect yourself against unexpected inflation, and then inflation moderates down to 0% in the next year. How are you going to explain to your investors that you lost 3% of their money?

Beware that, in the 2008 crisis, even some money-market funds lost money. The US money market funds have since been regulated so that they won't lose money in the same way, but I am sure more ways exist. Stay in government bills, insured bank deposits, and money-market funds that invest in government securities only. The extra 0.01% yield is not worth the risk.

The trust of your investors is more precious than the cash they gave you. If in 12-18 months things look good you won't have a problem raising more cash.