The office here does not offer support of extenders since they inevitably develop issues. That is, we may offer ideas we will never support them. You learn why after a few months of networking in the wild.
I can't see where the router is here. That is, it could be in the modem as it's IP is a dead giveaway it's more than a router.
TL;DR. Check that all firmware is up to date and set the units to reboot daily.

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