Past studies show that young star-forming galaxies are the primary sources driving the Epoch of Reionisation. Gas-phase metallicities trace the processes that enrich the interstellar medium (ISM) and reionise the intergalactic medium (IGM) in these galaxies. Thus, studying it can provide insights into the chemical enrichment of early galaxies and the reionisation of the Universe. However, accurately estimating the metallicities of high redshift galaxies is still an ongoing challenge. I will present new results using JWST/NIRCam wide-field slitless spectroscopy and imaging in several fields at 5 < z < 7 from the EIGER survey, COLA1 survey and the ABELL 2744 lensing cluster field from the ALT survey. To understand the evolution of the mass-metallicity relation at these redshifts, I perform stacking of rest-frame optical spectra of [OIII] emitting galaxies in different stellar mass bins. I will show the metallicity estimates for galaxies with masses in the range 10^6-10 Msun at these redshifts. I will also address the issue of degenerate metallicities obtained using various [OIII]5008/H-beta metallicity calibrations and show how we break this degeneracy through the temperature-sensitive [OIII]4363 emission line. With a large sample size of ~900 galaxies at these redshifts, I will present statistically robust results, which will help characterise the properties of the first galaxies and better comprehend their ISM.