Teradata SQL getting last not null value

I have a table in Teradata that looks like this:

Account_Num Install_Due_Dt  Install_Num Install_Pay_Dt
12805196    12/08/2019                1     12/08/2019
12805196    10/09/2019                2              ?
12805196    10/10/2019                3              ?
12805196    11/11/2019                4     13/09/2019
12805196    10/12/2019                5              ?

I need to fill the column Install_Pay_Dt with the first not null value. For example, it should look like this:

Account_Num Install_Due_Dt  Install_Num Install_Pay_Dt
12805196    12/08/2019                1     12/08/2019
12805196    10/09/2019                2     12/08/2019
12805196    10/10/2019                3     12/08/2019
12805196    11/11/2019                4     13/09/2019
12805196    10/12/2019                5     13/09/2019

I’m using Teradata 15 so I can’t use lag. I’ve been searching a lot but I can’t find a solution. The ID column is Account_Num and the order column is Install_num.

I’ve tried to do something like this:

coalesce(Install_Pay_Dt, MAX(lag_) 
OVER(PARTITION BY 1 ORDER BY Install_Num asc
ROWS BETWEEN 1 PRECEDING AND 1 PRECEDING)) as lag

But it only fills the second row.

Answer

last_value is quite similar to lag, both support the IGNORE NULLS option

last_value(Install_Pay_Dt IGNORE NULLS)
over (partition by Account_Num 
      order by Install_Num )