Sorry, but I would never install a 120mm AIO to an 8-core CPU like the 9700k (I have one myself). It isn't that the CPU is overly warm, it's just that a cheap AIO that uses a 120mm radiator can't dissipate much heat. However, that might not really be the real issue here.
I've measured flow rates from AIOs before and they're usually less than 1L per minute, if I recall correctly. Most good watercooling loops are rated at 0.50 to 1.0 gallons per minute.
You might have an airlock in the pump preventing coolant from flowing. With the pump running, thump the pump with your finger and wiggle the tubing, but be careful not to stress any fittings. You can also disconnect the radiator from the case and give it a shake to dislodge an airlock. If the tubing is warm but the radiator is cold, something isn't moving coolant and if the pump RPM is registering and you can feel 'pulsing' in the tubing, then you likely have an airlock and it needs to be dislodged.