All my sympathy to you. It sounds extremely challenging.
I think selling the ipad might be a thought -- buy yourself something nice!
I have girl a similar age, and I find days with the schedule you describe really hard in that it's a lot of time just at home. Are you able to look up more playgroups etc in your area? Where we live (London) there are free playgroups in children's centres and churches most weekdays. I would devote some time after bed to making up a schedule of all available activities. You say he cries when you take him out but will he keep crying? I reckon just try to steel yourself for the crying that comes when he is put in the pushchair and go out anyway. My daughter ALWAYS cries when she goes in the pushchair and I distract her by singing.
For weaning off the ipad/phone - I agree that this is a good idea, as it's hard for any other toy to compete. I think it will be massively easier if they are not in his eyeline. If he sees you on the phone and put it away he will want it. I suggest that for a week or two's cold turkey period you don't use the phone in front of him OR only use it eg in your bedroom, then leave it there, and come down to play, so he knows it won't be around at playtime.
I would also consider introducing some books as an alternative to the ipad. You can join the local library in your baby's name (where I am it's better to get book's out in the child's name as there are no fines if you bring them back late!). In my experience books are something you have to persevere with -- it might take a while before he sits down for a whole one (or he might never do it!). But if you find one that he takes an interest in the pictures of and persevere with putting in front of him and reading every day, over time with familiarity he will probably grow to enjoy it. The Very Hungry Caterpillar, Brown Bear Brown Bear What Do You See and other books by Eric Carle are very popular with that age group, so you could see if you can find those in the library (you will be able to reserve them if they are out). Otherwise any books with not too much writing (like a sentence a page is enough) - and make sure you point at the pictures when you see the words as he'll start to recognise the images.
Libraries are also a good place to just hang out -- your local children's library will have a story and singing session which I find good. If there's nothing on, the children's library will also have toys and a nice big space to crawl about in, and probably other mums will come in and you can have a chat!
You mention the toys you have and how he's not interested -- I would not worry to much about this. What my dd likes doing is just crawling round and exploring. I also wouldn't bother trying to 'teach' colours etc. By all means point at stuff and say 'oh look at the red bowl etc' but trying to 'teach' an 18 month old anything is only going to lead to madness as they definitely won't cooperate (and might pick up on your frustration too).
On sleep - my daughter is a bad napper too and tbh I often resort to pushing her round in the pram in order to get her to sleep. It's not ideal especially as it means you can't rest as he naps, but you could at least listen to music. Or you could try getting him to sleep in the pram and then going back home without waking him up.
I also wonder if you could do bedtime 30 mins earlier. Do you do a bath at bedtime? I find doing one every night lets my DD know it's going to be bedtime soon so it's not too much of a shock!
I mean the above thoughts to be helpful not patronising, please ignore if they don't chime with your experience. I agree with others that it's a very challenging time, and it's especially challenging for a single parent. You obviously love him very much and I think you are doing a great job in very difficult circumstances. It will get easier.