What does rig veda hymn no.121 verse 5 mean by By him heavens are strong and earth is stedfast , by him lights realm and skys vault are supported. Is this referring to the common myth that there solid dome above earth as found in bible and quran or does it mean something else due to mistranslation in english.
No, it should be read as a metaphor, it simply refers to everything being dependent upon God's will. See Bhagavad Gita chapter 10, especially verses 32, 39, and 42. In verse 32 Krishna says (Swami Nikhilananda translator):
Of created things I am the beginning and the end and also the middle, O Arjuna...
And verse 39:
And that which is the seed of all beings--that I am, O Arjuna. There is no being, whether moving or unmoving, that can exist without Me.
and verse 42:
...With a single fragment of Myself I stand supporting the whole universe.