How to set the Leased Time on the ADSL Router to forever?

Well, we can’t really answer this question unles you tell us the manufacturer and model number of the router.

However, I suspect that the answer is to set the lease time to 0 (zero).

@the100thmonkey The manufacturer and model number is Strabridge Linx 525.

@the100thmonkey is correct about setting this parameter to zero.

@Dr_Lawrence @the100thmonkey I tried setting it to 0(zero) but it says that” the parameter 0(zero) is out of range of [1–65535]” so, do I need to set it to 65535 instead? Because my leased time I have to set it in hour

The 65535 hours will give you approx 7.5 yrs before you need to worry about it again. Unless you are luckier than I am, you’ll be replacing it before then. So, yes, set it for the 65535.