Could be even the splitter quality. Have you checked yours? If your ADSL line does not have a phone line, try without the splitter. Have you tried another RJ11 cable? Also it could be the software issue on providers side.
I wonder, what if you record audio of connection, and just listen the times of disconnection for anomalies?
Is there any suggestion about a ADSL MODEM + ROUTER WIFI that is stable on the ADSL2+ line ?
I would choose separate ADSL modem, like some old blunt ZyXEL 660 — I had one in the past, when I was using ADSL line. So the ADSL connection + ppp link with authorization would be controlled by the modem itself, aside of your main router. And then ethernet cable from ADSL to your router's WAN port.