I'm from Indonesia and using Indihome (a state-owned company) as my Internet provider. When browsing the internet on any browsers or platforms, when you enter a website it would occasionally redirect users to its own page, which usually has some kind of advertisements. One time it even shows the ads page when I'm using steam client. The page would persist for around 10 seconds before it redirects the user back to what they're browsing before. If you don't want to wait, there is a skip button or you can just reenter the website address.
Changing ISP is not a feasible option, they have a monopoly on the market. Is there a way to skip these ads page?
This depends on how the ISP achieves this.
They either 1) manipulate your DNS queries; 2) inject/modify the HTTP(S) responses or 3) use a tool on your device to manipulate your browsing.
Start by enabling DNS over HTTPS: https://support.mozilla.org/en-US/kb/firefox-dns-over-https
If that does not solve the issue, then the ISP might alter the HTTP/HTTPS responses. It's hard to solve that issue. At least you could use a plugin like HTTPS-everywhere to use HTTPS sites where possible. HTTPS sites can only be manipulated when the client has a custom root certificate installed.
Make sure that the ISP did not install any 'tools' or 'helpers' on your device.