I’ve been going through some of this firm flaccid horror for the last 1.5 years. A cold and shriveled penis that even hurts if you do not massage it several times throughout the day to get some blood to it. Pills, herbs, and even testosterone aren’t helping, only new girlfriends, and only sometimes.
One thing that I found works like a temporary but very powerful vasodilator is alprostadil gel. It appears to be banned in the US now, but I had a stash of it since ~2012. It is meant to be injected to the inside of the urethra. Gross, but very effective.
The only currently available FDA approved form of alprostadil is called MUSE. It is sold as urethral suppositories. I have not tried it, but I do not like that its dosage appears to be fixed and non-adjustable.
Alprostadil injections are also possible, but way gross, unless this is your last resort. Then do it.
With my alprostadil gel stash running low, I have tried other methods. The only one that I found effective is very-very slow massage of the entire penis, from the perineum to the glans, through the clothes. A hand pressure through the clothes appears to dissipate more evenly and over a larger area, moving the blood more effectively up the shaft and to the glans. Do it in one direction, very slow but with a constant and steady hand pressure. Spend more time slowly near the perineum than the shaft. If you notice the glans slightly inflate as the end of each stroke, then you are doing it right. Do not overdo it. Rather, do it only for as long as it feels good, and do not exceed that pressure. This is just a massage, not a PE exercise.