Do you have any content blockers on your router/ISP? We set up a child portal on ours and it was blocking all sorts (including Google Classroom) that it shouldn't; even though stuff was whitelisted, it still kept doing it. Ended up binning it.
Other option is to check your DNS servers - try setting them to 8.8.8.8, 8.8.4.4, or 1.1.1.1 - if that works, it could be an issue with your router or your ISP's DNS configuration.