How to get MAX numeric value from a column with char values in MySql
If the numbers are all Integer, that you want

SELECT MAX(CAST(col AS SIGNED)) from nextg WHERE col REGEXP '^[0-9]+$';


Like @Akina said in the comments MySQL knows another way

SELECT MAX(col + 0) from nextg WHERE col REGEXP '^[0-9]+$';


See example

If you have also Decimal Numbers.

SELECT MAX(CAST(col AS DECIMAL(8,2))) from nextg WHERE col 
REGEXP '^[0-9]+\\.?[0-9]*$'