It's been a while since I had a TiVo, but go to video settings and run the test, untick all except 1080i, you will see 1080p below it, put a tick in there. If necessary reboot the TiVo.
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Hub 3.0, TP-Link Archer C8, TP-Link TL-SG1008D 8-port gigabit switch, V6