As a landlord, you are providing an essential service. The regulations allow you to travel in connection with this purpose.
The regulations also allow anyone to travel to obtain money. Now that was probably intended to cover going to a bank or atm but the law is what is written not what is intended. So, should you decide to travel to collect your rent, you're covered on either count.
If you can't, or don't want to, you should go ahead and serve a notice of arrears anyway. Serve a fresh one for each month missed. You can't proceed with the next step which is a notice of termination but you should make a start anyway.
Finally, do refer the case to the RTB dispute resolution service. They claim they are looking at how to provide an adjudication service in the current times. I wouldn't have great confidence in them but they're all you've got available to you. They are also offering telephone mediation. If your tenants are anyway reasonable, this might work. But if not, you'll eventually have to go through the RTB to evict them anyway. So you might as well get in the queue. It'll be crowded so get in early!