cash and accrual, you can't have the same income posted as both.

If the retainer is non refundable, then it is income when received. If the retainer is refundable, you fail to provide the service or the customer can call up and cancel, then the retainer is a liability and booked as deferred income.

QB is accrual accounting it reports on cash basis if you wish.

create a liability account named deferred revenue
create a service item named the for the service and link it to the deferred revenue account

use that item on the quarterly invoice
monthly do a journal entry, you can schedule it if you want, debit deferred income for the monthly amount, and credit service income