Having them teleport to the relevant port really is fine, IMO.
And what about the many problems inside the same land mass? For example the country-out-of-supply-after-surrounding-capital issue (Moscow for example), or country split on several isolated big areas (Spanish Civil War, China for example)?
Better to fix it correctly than to patch it hurriedly and non-satisfactorily.
Supplies, oil and resources generated and spread out all over the country, that is a natural solution, and thus will have no side effects.