I am surprised that it is actually nftables and not legacy iptables!

I am not sure how openwrt stores nftables config and how it looks, however you should find a `chain {}` block where `hook forward` is written, next lines within this block will specify policies regarding allowed traffic and there should be a rule lile `reject`

If it lools similar to what I have described then you need to add rules that will allow traffic to and from wireguard interface as it is currently allowed for your WAN interface (it might be eth0 for you)