Live steraming replication setup issue!

Hi,

Currently we have master -> slave -> DR hot standby streaming replication in current prod environment. Between master and slave server replication running fine. Between slave and DR server replication is broken and I am trying to fix it. For DR server, slave server is master server.

Issue: Few days back, DR was lagging behind slave server and stopped replication. I tried to setup the replication from slave to DR (currently there is replication running from master to slave) with pg_basebackup command, I am able to restart Postgres without any error on DR server but when I try to run any psql on DR, it throwing up below error.

psql: FATAL: the database system is starting up

Slave configuration:
hot_standby = on
listen_addresses = '*'
wal_level = hot_standby
wal_keep_segments = 3000
max_wal_senders = 5

default_statistics_target = 100
maintenance_work_mem = 1792MB
checkpoint_completion_target = 0.7
effective_cache_size = 22GB
work_mem = 144MB
wal_buffers = 16MB
checkpoint_segments = 32
shared_buffers = 7GB
max_connections = 300

DR Server configuration:
listen_addresses = '*'
hot_standby = on
wal_level = hot_standby

default_statistics_target = 100
maintenance_work_mem = 896MB
checkpoint_completion_target = 0.7
effective_cache_size = 10GB
work_mem = 352MB
wal_buffers = 16MB
checkpoint_segments = 32
shared_buffers = 3584MB
max_connections = 300

How do I setup replication between DR server and slave server while slave server and master server are running? I cannot stop master server. Can someone please guide with steps?

Thanks for your help in advance.

Thanks
-Ashish