Not able to whitelist "bs.serving-sys.com"

Hi there,
I have a Hotspot login page that serves Ads. As the login page contains Ads from bs.serving-sys.com, I tried to whitelist it under bs.serving-sys. However, the Ad still doesn’t go through and return error. I look at the wallgarden count, and it says 0. Do you guys face any problem like this? I tried google it and most of the sites are trying the block it. However, the link that I’m having is from a big company and it’s legit, it just serves Ad and collects Ad count.

I wonder if Mikrotik blocks this link by default. However, if I’m connected to the internet and allowed to surf any websites, the link actually works.

Thanks