ADSL Asus router dsl-n55u_c1 or dsl-ac68U which is more stable?

Any suggestion for replacement for my current TPLINK w8970v1 ADSL modem router replacement? Current W8970v1 starting unstable to hold the ADSL signal dropping from SNR 25 to 6-7 sometimes.
For ADSL modem broadcom chipset is better or ralink/mediatek?

Thank you.