I have same problem, my father is managing one of my rental units - asked my father many times to raise the rent since no raise for >10 yrs and tenants slyly changed without informing us and thus skipped the normal price hike in between tenants.

Last year I beh tahan, called tenant and said bank interest rates hiked I no longer can cover the bank instalments, need to raise rental 20%. 20% raise after >10 yrs, the tenant say even after raise, it's still below market rate, so he willingly accept. Actually bank instalment is only slightly more than 50% of the new rental - so ok la.