Quick take
Start with feed pressure, filter age, membrane rejection and drain routing. A noticeable performance drop after months of use is more often a maintenance issue than proof the entire unit failed.
Low faucet flow
Loaded prefilters, low inlet pressure or membrane restriction can reduce flow. Check whether the decline was gradual or sudden.
High TDS
Use rejection percentage rather than a single TDS number. A new membrane may also need adequate flushing before readings stabilize.
Pump noise
Some pump sound is normal in tankless RO. New rattling, grinding or vibration should prompt a mounting and support check.
Filter replacement
Use the correct ASR-series replacement schedule and reset indicators only after filters are actually replaced.