Where to advertise PhD positions without paying a fee?
Euraxess may be an option. Its openings are quite heterogeneous (sciences and humanities). Despite the name, it is not restricted to European countries.

There are other free scholarship aggregators like scholarship-positions.com. If you search academia.stackexchange you may be able to find others.

Also, you may be able to find reddit channels associated to your area. At least in computer science it is not uncommon to see such openings posted there.

Another option is trying to locate a google group where other labs in your field post their openings.