Yes, very high CRC’s but not showing REIN or SHINE values ?
Try an older firmware to see if CRC problem is the same and factory reset and build from scratch (not backup) after loading firmware – Version 1.1.2.3_617
https://dlcdnets.asus.com/pub/ASUS/wireless/DSL-AC750/FW_DSL-AC750_1123617.ZIP
If the above does not work, can you post a Traffic Manager – Spectrum screenshot – I think the DSL-AC750 support this option ?
Also try a new DSL cable between modem and wall socket.
The DSL-AC750 has I think a setting called ESNP (Enhanced Sudden Noise Protection) make sure this is On ?
Not sure if this will also work in your region of the world, if you use a landline telephone, look up how to do a quiet-line-test and see if you can hear any crackling / hissing on your telephone ?
When u Called your ISP did they say there was a line fault ?
If still high CRC’s, then probable a faulty electrolytic capacitors in the Modem which is not good and time to buy a new (better) Broadcom Modem Router as yours is MediaTek and very sensitive 
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In a completely sane world, madness is the only freedom (J.G.Ballard).
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