how to use 受欢迎 in the past tense?
you can also use “从前”，“过去”，“曾经” to indicate the past time, but both imply that the current situation is opposite of the previous. 

This restaurant used to be popular among foreigners(,but now is not that popular).


这家餐厅从前很受外国人的欢迎（，但现在已经不那么受欢迎了）。 
这家餐厅过去很受外国人的欢迎（，但现在已经不那么受欢迎了）。
这家餐厅曾经很受外国人的欢迎（，但现在已经不那么受欢迎了）。


if you want to use "已经" to express "have been", you can say:

This restaurant has been (very) popular among foreigners.


这家餐厅已经很受外国人的欢迎了。