What are you using as the CPE? If it's Mikrotik, I think you could solve this by using EOIP tunneling between the CPE and the NAS.I have 1 client radio on an office building with multiple customers.
I have the radio plugged into a hub then from the hub it goes to each customers router and the routers are using PPPoe to auth.
This was working fine when I had only 1 customer using pppoe and 1 using hotspot to login, I added a 3rd customer and they will use pppoe. But now the ones using pppoe keep lossing there connection. 1 may work for a few hours and the other for a few minutes and they keep reconnecting.