I use a Buck/Boost Module for my Camper/Motorhome.
They are available for a reasonable price (think insurance) on various auction sites. You need to make sure it is capable of supplying adequate current for the Router. This description should lead to a 3A version. I do protect the supply and output side with a fuse. I have run my newer GL-X750V2 and previously, a DWR921 from this.
3.5-28V to 1.25-26V DC-DC Converter Boost Buck Step Up Step Down Voltage Module
Disclaimer: This is just what I use, so far, I have not had a problem. But…