Split rims are OK if installed or assembled correctly. Assuming this is done because they are on the trailer and being used. The next tire change you will want to buy new rims and new tires.
Or buy rims in anticipation of the new tires.
Driving your trailer you would want to have a spare that is NOT a split rim. Such that if a tire on a split rim fails you have a non-split rim to be placed in that locate. This would be a strategy if you want to own the trailer on a budget now and spend the cash in other areas. If you take this strategy the goal is to have at least one non-split rim and tire available to be used in the event that a tire fails that is mounted on a split rim.
If you have the cash now, get 5 new rims and tires.
Which ever strategy you take, you will eventually be replacing the split rims. The timing of that will depend on how you have cash allocated.
Assuming this is a new to you trailer, do inspect the age of the tires. Most of us are trained in replacing tires that have worn out tread. (Especially on daily driver vehicles) However tires have a shelf life based on age. That age limit has differing opinions. And if you have a trailer that has tires that are perhaps decades old, replacement is needed even if the tread looks good. In my opinion the tire age limit is reached at 7 or 8 years. This is just my non-professional opinion. Other will disagree with me.
1966 Mercury Park Lane 4 DR Breezeway 410 4V, C-6, 2.80 - Streamless.
1966 Lincoln 4 door Convertible 462 4V 1971 Ford LTD Convertible 429 4V Phoenix ~ Yeah it's hot however it's a dry heat!