ActiveRecord::ConnectionNotEstablished: No connection pool with 'primary' found for the 'reading' role on staging

I'm working on migrating our current Rails application(v 6.0.3.4) to a multi-tenant app where each client will have their own DB. The code works fine on my local environment but it gives ActiveRecord::ConnectionNotEstablished: No connection pool with 'primary' found for the 'reading' role error on staging.

I'm selecting the DB at the rack level depending on the subdomain.

module Middlewares
  class Multitenancy
    def initialize(app)
      @app = app
    end
 
    def call(env)
      @env = env
      
      database = { writing: "primary_#{subdomain}".to_sym, reading: "replica_reader_#{subdomain}".to_sym }
      

      ActiveRecord::Base.connected_to(database: database) do
        @app.call(env)
      end
    end

    private

    def subdomain
       ActionDispatch::Http::URL.extract_subdomain(@env['HTTP_HOST'], 1) unless @env['HTTP_HOST'].nil?
    end
  end
end

database.yml

default: &default
  adapter: mysql2
  encoding: utf8
#  reconnect: false
  host: <%= ENV.fetch("MYSQL_HOST") { '127.0.0.1' } %> # 127.0.0.1 to force use tcp ip instead of unix socket
  database:  <%= ENV.fetch("MYSQL_DB") { 'development' } %>
  pool:  <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } * 30 %>
  username: <%= ENV.fetch("MYSQL_USER") { 'root' } %>
  password: <%= ENV.fetch("MYSQL_PASSWORD") { '' } %>
  port: <%= ENV.fetch("MYSQL_PORT") { '3306' } %>
  variables:
    sql_mode: TRADITIONAL

staging:
  primary_subdomain_1:
    <<: *default
    host: <%= ENV.fetch("MYSQL_HOST") %>
  replica_reader_subdomain_1:
    <<: *default
    host: <%= ENV.fetch("MYSQL_HOST_REPLICA") %>
    replica: true
  primary_subdomain_2:
    <<: *default
    host: %= ENV.fetch("MYSQL_HOST2") %>
  replica_reader_subdomain_2:
    <<: *default
    host: %= ENV.fetch("MYSQL_HOST2_REPLICA") %>
    replica: true
``