Just for the records if anyone has problems with wireguard (not starting the interface/no connection) on clean openwrt: Wireguard doesn´t start / connect when kernel entropy is too low. An my device (AR300M) it took some time after system start to get enough entropy to start wireguard. Apparently there is no error message when entropy is too low. Wireguard simply doesn´t work…
Too low values will prevent wireguard from establishing a connection. To speed up this process I installed the “haveged” package under openwrt. Now wireguard connections are established directly after system boot.
May be this hint helps other wireguard users.