It's not up to Whatsminer to accept or deny the access, if you can locally access the miner on port 80 then you should be able to do so remotely, I believe your router could be blocking you because Whatsminer GUI has an issue with CA root cerficiate, most firewalls will flag it so this could be the reason why, you do to need to play with your router security settings which isn't something I would recommend, port forwarding to your miners is risky and things could go wrong.
You really need an extra PC on-site to be somehow on the safer side, it doesn't need to be a fancy one, get the cheapest one you can, I am sure you can find something for $100 or so, if you can't afford it, wait until you can.