How to upgrade Unity Ads to 3.5.0 for Unity2018.4?

[rockwjx_unity]

I am working for a project which is running on Unity2018.4.2f1. When I upgraded the sdk to 3.5.0 by Package Manager for the iOS14 changes, there was an error:

An error occurred while resolving packages:
Package com.unity.ads@3.5.0 has invalid dependencies:
com.unity.ugui: Package [com.unity.ugui@1.0.0] cannot be found

I tried to add the

“com.unity.ugui”: “1.0.0”,

to Packages/manifest.json but the same error was shown

Package [com.unity.ugui@1.0.0] cannot be found

It seems that we have to upgrade the unity to 2019, but this is hard for us now.
So my question is how can we upgrade Unity Ads to 3.5.0 for Unity2018.4?
or, is there a solution for the iOS14 changes with Unity2018.4?

Thank you

[kyle-unity]

You can also resolve this by upgrading to 2018.4.18f1 or higher. This should be easier than upgrading to 2019

[rockwjx_unity]

I came out that we can download the sdk from AssetStore instead of PackageManager.
Thanks anyway