@rustam54
You are saying very right. He needs a regulated dc power supply for the modem. Best way is to get SMPS ( Switch Mode Power Supply). Most current adapters and chargers are of this nature. These can operate at 100 - 250 VAC, They even don't care about frequencies either 50Hz or 60Hz. These new types of adapters are everywhere , laptops, cameras, mobiles, routers ( if its PTCL then they don't give you one).
These new SMPS gives us world wide freedom. Only we need plug converter for that country. These Power supplies are totally different and smart. Initially these were so expensive. Still they are. For example your laptop charger.
These power supplies are fault tolerant, overheat protected, short circuit protected.
Best way to sort this issue is to get Netgear adapter from landa (used item). 12V DC or 9 V DC with 1 Amp standard.
I have a modem+router whose adapter is 12VAC. It has whole power circuit inside. This is the best router Which I have so far.