I run a Unifi USG-1 router/gateway now but before that I got a used AC1000 router from the thrift store for $5 and loaded OpenWRT on that. I eventually got an Aris modem to replace the Comcrap Gateway because it was messing with the DNS traffic, even when piHole was set as DNS for every machine. So if I were you I would go look at the OpenWRT list of units that will accept their firmware. Pay attention to the hardware revision as well as the model number, it matters.

https://openwrt.org/toh/views/toh_fwdownload

Then head over to a local thrift store or two and see if you can find any for cheep.

I think this one is a custom module, it solders into a slot on the main board vertically with pads on both sides of the modules bottom edge.

https://fccid.io/2AB2Q-LA02301/Users-Manual/11-User-Manual-4810925

Edit: fixed link

I also have a VPN set up to my home network that I can connect to with my mobile devices. once connected I use the HA mobile app pathed to my local HA server IP. I don’t use HA cloud or any other cloud service. It works well for me, I obviously can’t get notification on the mobile when not connected but I have not had a need yet. If/When I’m do want notifications I would most likely explore, e-mail, e-mail to SMS, SMS API or something that won’t involve external servers. I would not want to send sensitive info via SMS but a shot alert that said there is something to look at like “HA Alert: Motion detected cam 1” or such would be fine.

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I just got a Filohome (tuya) dual switch and used tuya-cloud cutter put ESPHome on it. When setup with ESPhome it will still work even if HA is unreachable ok WiFi is down. I never even downloaded their app, made an account, or signed in.

Note for anyone trying to do the same, the Filohome triple switch device in tuya-cloudcutter will work to load the initial boot firmware but you need to change the yaml config when you install ESPHome. If anyone wants the yaml file let me know.