Can you pull cable to there?

Else you would need to consider point to point system.

Try access it using mobile data.

Local will work because it is your network. Outside of that depends on ISP.

Does it say any error when trying to access? web browser?

Tried to delete the wifi on iPhone and reconnect again?

It’s useful for local transfer between computer like NAS. You do need to upgrade other equipment to 10G as well.

Did you setup router 2 as Access Point mode and plug the cable from router 1 to LAN port, not WAN port router 2?

4K streaming is around 25Mbps, so x2 = 50Mbps, still doable.

I would connect it to main router first, in case something happen to MR2200AC, wired device still have internet.

Do your phone/laptop support Wifi 6? For higher max speed, you will need to use higher bandwidth like 80Mhz or 160MHz, assuming your device support and not much interference.

Can you check if this turned on? https://support.norton.com/sp/en/us/home/current/solutions/v138204009

Disable the DHCP on cAP AX?

If you just want to test if pihole is working or not, you could set manual dns on the device.

If want to wired them 2.5Gbps as well, Asus ZenWiFi Pro ET12?

Is there ethernet port/cable or coax on the other side?

Try readjust the other Deco location, you can’t put too far if its wireless backhaul.

It does if you enable the share in router. Can check the tutorial here https://www.youtube.com/watch?v=Q2sLtVdZ5fI

like hamachi?

No, Access Point would extend the connection from initial router.

it will be compatible, run at 1Gbps

Set the new router as Access Point mode, plug the ethernet to LAN port, not WAN.

seems can go more than 2.5Gbps https://redd.it/121sxom