@Ethan-TP Thanks for your response. It doesn't occur when my PC is directly connected to the modem-router. It also doesn't happen when I use an old TP-Link router. So I conclude it has something to...

Since I use the ER605 a lot of websites start loading after a long while or not at all. When the do load everything is fine. I thought that it could be a DNS problem, so I tried using another DNS...