ORBI RBK50 cannot get internet connection from D6200

Brand new ORBI RBK50 (to fix poor wifi across two-storey townhouse), turning Netgear D6200 into a modem only ... stalls because no internet connection can be established.

Had internet ex the D6200 via ethernet and wifi before I started setting up the ORBI.  I set the D6200 to modem only (via computer), connected the ORBI via ethernet cable to the D6200 (ethernet 1 on the D6200 to internet on the ORBI), power on, wifi ex the ORBI fine ... just no internet.

Followed all prompts in the setup routine (powered off/on the D6200, checked all connections ... trying all of the ethernet out ports on the D6200) but no internet.

Internet from the D6200 via ethernet to computer works on reset for a short period then disconnects.  Cannot get back into the D6200 to turn back to modem plus router (to restore wifi to where I was before I started).

Any help most welcome.  Not exactly dad of the year here ... "I'll improve the wifi for you", and then no wifi.

ps. tried 3 ethernet cables between the D6200 and ORBI