Sorry, we didn't run into this issue, but if the embassy said that you will be able to proceed with the payment after visa appointments are made available, then I would go with their advice! It sounds like it's a(nother) waiting period, but once they open up slots then you can generate the payment slip through your online account and then pay in-person at Abyssinia Bank.
Here is the official instructions from the embassy in Addis, but it outlines the same process. (make an account --> download and print the payment instructions --> pay in person --> schedule your interview). https://travel.state.gov/content/dam/visas/K1/ADD_Addis-Ababa.pdf