Sewer systems (plural) act as a portal (singular)? I don’t think so… Change a portal to portals.
Change the comma after locations to a period and begin a new sentence with The government.
In UK English, groups are construed as plural, so the verb have is correct with government here. (Many of the examples I use come from a blogger in the UK who shares writing prompts she gets from her followers.)
Add a comma after The government.
Change 2 to two.
Sewer systems act as portals to different locations. The government, however, have made them illegal. You have been hired to deliver a package from the US to Australia in under two hours.