How do you test an MVP when there is the "Chicken or the Egg" problem?

I am almost finished with an MVP/beta version of a SaaS that helps businesses of specific industry market to potential customers. While I believe the service will be considered valuable to both the companies that sign up and their potential customers, I really don't know how to proceed with a proper way to validate this hypothesis.

For me to find truly test whether or not my customers (the businesses) are willing to pay for the service, I need users otherwise it is hard to argue a return on the investment. On the flipside, I can't get users without businesses signed up.

I'm considering offering a 3 - 6 month beta period that would be free to companies. The problem I have with this is that it's not really testing the viability of the service. Moreover, I won't be able to work on it full time since I do not have investors and I am afraid of falling short on bug fixes, feature releases, and most importantly, marketing to end-users. I could probably seek out investors but this is another chicken and egg situation; I will not have truly tested the service's viability to investors.

How does one tackle the dilemma of having two customer bases that are dependent on one another?

Bonus question: If it is to proceed with the beta period, would you seek out investors? If so, at what point?

Thanks, Chance (long time lurker, first time poster)