Custody of children

I have a few questions. My one son is 7 and the other is 2 and the mother has taken them away from me and moved far away where I cannot get to see my kids. The mother has moved away with both kids and is currently staying at her parents they are staying 400km from me and I find it hard to travel all the time its a costly factor as well and I do work and also the last I have seen my kids was 4 month ago. She took the kids and moved away without my permission. Majority of the day she's out of the house at work like 9 to 10 hours a day which her mum feeds, dress and take the kids to school. She is also working with non mahrams and likes to socialise with them. She also banned me from going down and seeing my kids and from phoning them. I'm not sure I can't judge but maybe she can be also involved with someone. She's telling people that she can't give me custody of my 7 year old son because there is no one to take care of him as my mother is old 75 years of age but I know my mother will take care of him because she is looking after other 7 grandchildren she feeds them take care of them some are also married today and there's also my elder nieces that are at home so when I'm at work there's always elders at home. So mufti saab how do I go about all this? If I remarry what will be my custody? The mother sits on her phone the whole time whatsapp, bbm, twitter, wechat and has very bad influencing people, friends which are divorced as well. They are far away from me 400 km distance.