ADSL2 is a second generation ADSL spec. ADSL2 offers better performance over marginal lines and other minor improvements. All that is done within the modem itself has no affect on router.
Since speed is low using wired Ethernet you have eliminated WiFi as the problem. What speed did you have with Cable? My first guess would be the router is underpowered. But Cable is typically faster then DSL so that is unlikely.
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