Nintendo Switch w/NOK 5G21 Gateway

I was able to switch our gateway to the newer KDV21 a few days ago, in hopes to resolve my son's issue that everyone here is having with connecting to online game plan on his Super Smash Bros game on Nintendo Online. As soon as we setup the new gateway he was finally able to connect to other online players on Super Smash Bros. It was a laggy but it worked. He tried the next day and got the error again: "Error code: 2618-0513 Unable to connect to other consoles. Please try again later.". We don't know what's going on. He was able to connect the other day and now he can't. We tried to move his Switch around the house to get a better signal, even right next to the gateway, with no success. Right now my husband it out getting an ethernet adapter to see if we hard wired the Switch to the gateway, if that would work. What do you guys think? Do any of you think that will work or do you have any other suggestions. We greatly appreciate all recommendations. ¡Gracias!