Erlang / Elixir are languages that are easy to begin learning, but difficult to master. In our case, the team was able to get the basics up and running quickly, but the service became increasingly difficult to manage as our product requirements expanded beyond the strengths of the Erlang ecosystem's core competencies.
In our case, the original team of developers was excited to write a greenfield service in Erlang, but much less excited to maintain and scale it long-term. Some of them used their experience to speak at Erlang conferences, which led to them being recruited away by companies who were desperate for Erlang developers to maintain their legacy code after the core team left. Ironic.
At first we tried to train existing engineers on Erlang. Doesn't work. Not because Erlang is bad, but because people don't want their companies changing the direction of their career development just to support someone else's legacy project that requires an on-call team to support.
So your two options are: Hire at least 2, preferably 3 Erlang engineers for the reasons discussed above. Harder than it sounds, because AFAICT there are a lot of companies with 2010-era Erlang backends that made sense at the time but now require dedicated maintenance engineers.
Or you can rewrite it in a more common language, which is the route we chose. Rewrites are a difficult decision for well-known reasons, but ultimately it was the correct choice. Being able to drop 10+ engineers into a project as needed is a huge accelerator compared to funneling 100% of your work through the 2 Erlang gurus in the corner who maintain a codebase that no one else wants to touch.