1. You need to choose UniFi VLAN profile in the Deco app. Video example below
2. Did you type in the UniFi username correctly? Example: blabla@unifi
3. Do you have the PPPoE password too? If not, you need to get it from UniFi helpline
4. Deco E4R is the lowest of the low. You might get bigger coverage but any device connected the secondary mesh/tertiary mesh will get very low speeds unless you connect all MESH device using Ethernet cable

Issue fixed?

Yes those steps will allow you to bypass the original router (IE. You no longer need the original router)
Afterwards you can remove the original router and you dont have to deal with multiple WIiFi SSID anymore

If by WiFi points you mean WiFi names (the correct term is SSID [Service Set Identifier]), then yes

For example, your original router broadcasts "moleman4eva@unifi" and your MESH system is broadcasting "moleman4evaMESH", with the VLAN configuration and PPPoE setup on your Deco E4 will eliminate "moleman4eva@unifi" WiFi signal since you'll be removing the original router from the whole network

There's absolutely no reason to keep the original router anyway. You're flooding your neighborhood with multiple WiFi SSIDs and wasting electricity (not that a router consumes much power anyway)