Update using subquery

I have SELECT query as below that need to be inserted into table wifi_user_analytic_dy.

SELECT
  date(acctstarttime)
, round(avg(acctsessiontime),2)average_connected_ap 
, calledstationid 
FROM ( SELECT acctstarttime, acctsessiontime, calledstationid 
   FROM wifi_radacct 
   WHERE acctstoptime <> '' 
   and date(acctstarttime)= date_trunc('day', now()) -'1 day'::interval 
 )a 
group by calledstationid, date(acctstarttime)

Sample result based on the query :

| date       | average_connected_ap | calledstationid |
| 2017-12-06 | 1000                 | publica         |
| 2017-12-06 |  800                 | tumis           |
| 2017-12-06 |  500                 | penyet          |

The above query is going to find average connection base on date and ap and update into table wifi_user_analytic_dy. Below is the expected result inside table wifi_user_analytic_dy.

| date       |  ap      | average_connected_duration_ap |
| 2017-12-06 | publica  |     1000                      |
| 2017-12-06 | tumis    |      800                      |
| 2017-12-06 |  penyet  |      500                      |

I am trying copy average_connected_ap data from the SELECT query above and update the table wifi_user_analytic_dy at column average_connected_duration_ap. The UPDATE query below is not produced the correct result. It is copying the same value to all ap.

How can I correct this update query.

UPDATE wifi_user_analytic_dy B
SET avg_connected_duration_ap = ( select round(avg(wifi_radacct.acctsessiontime),2)
       from wifi_radacct
       where date(acctstarttime)= date_trunc('day', now()) -'1 day'::interval 
       )
FROM wifi_radacct A
WHERE A.calledstationid=B.ap and A.date(acctstarttime)= B.date
;

Hope someone can advise me.