Вы можете сделать:
rt_cert <- paste0("PATH_OF_ROOT_CERTIFICATE/root-ca.crt")
cl_cert <- paste0("PATH_OF_ROOT_CERTIFICATE/xxx.crt")
cl_key <- paste0("PATH_OF_ROOT_CERTIFICATE/xxx.key")
con <- dbConnect(drv = RPostgres::Postgres(),
dbname = 'DATABASE_NAME',
host = 'HOST', # i.e. 'ec2-54-83-201-96.compute-1.amazonaws.com'
port = 5432, # or any other port specified by your DBA
user = 'USERNAME',
password = 'PASSWORD',
sslmode = 'require',
sslrootcert = rt_cert,
sslcert = cl_cert,
sslkey = cl_key)