I dose Iodum and Strontium as recommended (1 drop/100 litres of water) but I can’t get the levels up to minimum. What do you recommend?

You should try to slowly increase the dosage of both elements and monitor water parameters. Be careful though, it’s very easy to overdose microelements while using a highly concentrated products. Moreover, iodine is a volatile element, so it needs constant supplementation even after reaching proper level of it.