@Emix86 Check the negotiated speed between your main router and the RE700X. This could be done from your main router WebGUI. If the speed is 100mbps that's the culprit. You may change the main router...

@Memo2001 Check this thread. This could be due to the network protection against cross-site request forgery (CSRF) attacks - HTTP Referer Head Check feature enabled by default. Some TP-Link devices...

@chander123 Here's a link to download FW v1.0.16 for EU region. You can do a manual FW update. If your FW region is US (that's on your device bottom sticker) then try this link.

@martri This thread is the best source of information regarding REs Easymesh ethernet backhaul support.

@OMS_12 Check this thread regarding your issue (I saw you've already done that).

@tapo_user2022 OpenWRT may work for you if it's applicable for AX23 but it will void your warranty. Here are the routers that could do this job for you - ER605 in your case will suffice.