You play many roles within your Solo 401k plan. You are plan administrator, plan trustee, and plan participant.
You also play various roles within your 401k trust. First, let's define a few terms:
Settlor: The person or entity that establishes the trust (you and/or your business)
Grantor: The individual or entity that funds the trust by contributing assets (you and/or your business).
Protector: The individual or entity that watches over the trust.
Therefore, the settler, grantor, and protector of your trust is generally considered to be you or your business.
If a bank or exchange asks who fulfills these roles in your 401k trust, you can enter your name or your business name.