Even if the SYN packets were being ignored, the connection would still be unusable if there’s enough incoming traffic for most legitimate packets to get dropped. And as mentioned in other comments, the router in question is a shitty ISP router which can’t be replaced (although I do have a much fancier router with OpenWRT running behind that).
Get a router that has flood protection? This is like… Extremely basic network protection.
OpenWRT has had configurable syn-flood protection (enabled by default) since like 2010.
Even if the SYN packets were being ignored, the connection would still be unusable if there’s enough incoming traffic for most legitimate packets to get dropped. And as mentioned in other comments, the router in question is a shitty ISP router which can’t be replaced (although I do have a much fancier router with OpenWRT running behind that).