We used Makaton sign language for some real basic signing very early on. The hope was that it would help bring on communication as another form other than talking. It worked to some degree, but MIles' motorskill then was so bad, he only mastered a handful of signs. It was worth it though.
Here in the UK we have an amazing TV programme called Something Special. YouTube it. The programme is a childs show where the host (Justin Fletcher) will introduce one Makaton sign per episode. Its great for that, and the kids love the programme as well.

Yes my son and I used ASL and immersed ourselves with deaf community. Amazing experience. It really helped his receptive skills. He learned to use gestures and body language for words that required manual dexterity. He would use his body to act out what he was trying to communicate. I had to learn ASL and used sim com and psl (meaning I spoke while I signed using asl signs with English language dropping the articles and is, were etc) but it didn’t stop there, he started making sounds as we started signing. Then soon we required him to sign before he got something and then eventually say and sign it and soon he got it when he said it. He is fully speaking in sentences of up to 5-7 syllables as he has Apraxia as well. He has functional conversation.

It takes work to learn it but it’s more work in the long run to a have a nonverbal child. A short time of investing in this is worth a lifetime of worrying about your nonverbal child. Yes a child may not have the fine motor skills but many asl signs can be adapted to something easy for them to do. Asl is not meant for them to use as a permanent language but enough for them to understand what language is about which is a give and take of a message. So you give the kid about 100 to 200 words in asl that they know receptively and expressively and they start conversing more. The understand this messaging system . Also the imitation part . Use ASl as part of their non vocal fine motor imitation program. Say “Do this “and make asl letter with your fingers or sign and manipulate their fingers for a full physical prompt. Then slowly decrease level of prompt. You can always do close approximations. I signed on my sons body like the abc and signed different things on his body so he physically felt it on his back, chest , leg too

The thing about sign is that you have to be really dedicated to it. In other words, you have to learn it also. You might also look at Tap To Talk for an app. We thought about sign when my boy was younger but found it too difficult to dedicate the time to learning it.

We use proloquo (sp?) It is an amazing picture to word app. It has taken away so much of her frustration.