When you are sure the SH2 is in modem mode, switch it off. Power on the Orbi and make sure it is set to Automatic IP and leave it on.
Connect an ethernet cable from the WAN port on the Orbi to an ethernet port on the SH2. Note there should only be one ethernet cable attached to the SH2 in modem mode.
Once done, switch on the SH2 and this should work.
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